Vernier-tuned distributed bragg reflector laser

Abstract

The present disclosure relates to Vernier-tuned distributed Bragg reflector lasers. In some embodiments, a wavelength-tunable semiconductor laser includes a first mirror having a first integer number of repeating units, a second mirror having a second integer number of repeating units, and a gain section disposed between the first mirror and the second mirror. In some embodiments, each of the first integer number of repeating units includes a plurality of phase grating elements having a first plurality of pi phase shifts distributed among the plurality of phase grating elements, and each of the second integer number of repeating units includes a plurality of phase grating elements having a second plurality of pi phase shifts distributed among the plurality of phase grating elements. The first plurality of pi phase shifts and the second plurality of pi phase shifts differ by one phase shift per repeating unit.

Description

Cross-references to related applications

[0001] This patent application claims priority to U.S. Provisional Patent Application No. 63 / 775,032, filed March 20, 2025, entitled "Vernier-Tuned Distributed Bragg Reflector Laser". The disclosure of the prior application is considered part of this patent application and is incorporated herein by reference. Technical Field

[0002] This disclosure generally relates to tunable lasers, and more particularly to wavelength-tunable semiconductor lasers. Background Technology

[0003] A tunable laser is a laser that has an emission wavelength that can typically be adjusted during operation. In some cases, a tunable laser can operate at a single frequency, where the emission linewidth is very narrow, corresponding to a well-defined wavelength. For example, in optical communication applications, a single-mode or single-frequency laser with a large tuning range can be used to cover the entire band. Other tunable lasers operate simultaneously on multiple resonator modes, resulting in a spectrum exhibiting several or even many spectral lines. In this case, wavelength tuning typically involves shifting the envelope of the spectrum. Wavelength-tunable lasers have a variety of applications, such as fiber optic communications with wavelength division multiplexing, where a tunable diode laser can be used as a backup laser in the event of a failure of a fixed-wavelength laser. In other examples, wavelength-tunable lasers can be used to record absorption spectra in laser absorption spectroscopy, to record wavelengths specific to a substance to be monitored in a laser detection and radar (LIDAR) system, or to precisely tune the laser wavelength at or near an atomic resonance in laser cooling systems or laser isotope separation, among other examples. Summary of the Invention

[0004] In some embodiments, the wavelength-tunable semiconductor laser includes: a first mirror comprising a first integer number of repeating units, wherein each repeating unit comprises a plurality of phase grating elements having a first plurality of π phase shifts distributed among the plurality of phase grating elements; a second mirror comprising a second integer number of repeating units, wherein each repeating unit comprises a plurality of phase grating elements having a second plurality of π phase shifts distributed among the plurality of phase grating elements; and a gain portion disposed between the first mirror and the second mirror, wherein the first plurality of π phase shifts and the second plurality of π phase shifts differ by one phase shift per repeating unit.

[0005] In some embodiments, the optical device includes: a first distributed Bragg reflector (DBR) grating comprising a first plurality of repeating units, each of the first plurality of repeating units comprising a plurality of phase grating elements and a plurality of π-phase shifts, wherein: in each of the plurality of repeating units of the first DBR grating, the number of phase grating elements is substantially the same, and in each of the plurality of repeating units of the first DBR grating, the number or position of the plurality of π-phase shifts is substantially the same; and a second DBR grating comprising a second plurality of repeating units, each of the second plurality of repeating units comprising a plurality of phase grating elements and a plurality of π-phase shifts, wherein: in each of the plurality of repeating units of the second DBR grating, the number of phase grating elements is substantially the same, and in each of the plurality of repeating units of the second DBR grating, the number or position of the plurality of π-phase shifts is substantially the same, and the number of the plurality of π-phase shifts in each of the first plurality of repeating units of the first DBR grating is different from the number of the plurality of π-phase shifts in each of the second plurality of repeating units of the second DBR grating.

[0006] In some embodiments, the method includes: generating light through a gain portion of a wavelength-tunable semiconductor laser; reflecting the light through a first mirror comprising a first integer number of repeating units, wherein each repeating unit of the first integer number of repeating units comprises a plurality of phase grating elements having a first plurality of π phase shifts distributed among the plurality of phase grating elements; and outputting a first portion of the light through a second mirror comprising a second integer number of repeating units and reflecting a second portion of the light, wherein each repeating unit of the second integer number of repeating units comprises a plurality of phase grating elements having a second plurality of π phase shifts distributed among the plurality of phase grating elements, wherein the first plurality of π phase shifts and the second plurality of π phase shifts differ by one phase shift per repeating unit. Attached Figure Description

[0007] Figure 1 The illustration shows an example tunable semiconductor laser, which includes a gain region between a pair of mirrors and a filter disposed between the gain region and one of the mirrors.

[0008] Figure 2 The illustration shows an example distributed Bragg reflector (DBR) laser and an example related to the tuning range for the DBR laser.

[0009] Figure 3 The illustration shows an example sampled grating DBR laser, along with examples indicating the reflectivity of the front and rear mirrors and laser emission at various wavelengths.

[0010] Figure 4 The illustration shows an example related to the tuning range, which can be determined based on the mirror peak spacing and the difference between the front mirror spacing and the rear mirror spacing.

[0011] Figure 5 An example related to grating modulation functionality for an ideal comb reflector is illustrated.

[0012] Figures 6A-6E An example of a phase grating-based vernier-tuned DBR laser with improved supermode suppression manufacturing tolerances according to some embodiments described herein is illustrated.

[0013] Figure 7 The illustration shows an example process for operating a wavelength-tunable laser. Detailed Implementation

[0014] The following detailed description of the exemplary embodiments is with reference to the accompanying drawings. The same reference numerals in different drawings may identify the same or similar elements.

[0015] In various applications such as telecommunications, tunable lasers should have a tuning range that satisfies the full communication bandwidth with a high side-mode rejection ratio (SMSR). However, tunable semiconductor lasers formed solely by electrically injected gain regions patterned into optical waveguides with cleaved and / or coated facets have a wide gain bandwidth that limits the ability to lasing at a single frequency (or longitudinal mode). In contrast, laser cavities support multiple longitudinal modes, each associated with a corresponding resonant wavelength or frequency, and the light waves recirculated within the cavity are constructively superimposed for each round trip. Therefore, to select a single longitudinal (cavity) mode with high SMSR, the laser cavity can incorporate narrow passband filtering elements, such as mirrors employing gratings etched into passive semiconductor waveguides to reflect only narrow band wavelengths, thus providing filtering and light reflection in a single element. In such a laser design, the mirror providing combined filtering and light reflection functions includes a waveguide in which a grating is patterned, wherein the waveguide has an optical reflectivity band that can be tuned by carrier injection or Joule heating to position the light reflection band near one end of the tuning range. In addition to the mirror, the laser includes a gain section (where the light is generated) controlled by electrical contacts for injecting current into a suitably designed active region to provide optical gain; and a phase section with control terminals that allows for fine tuning of the longitudinal mode spectrum. However, carrier injection or thermal tuning cannot physically produce a sufficient refractive index shift to achieve full-band tunability.

[0016] Therefore, to overcome the limitations of refractive index tuning, achieve full-band tunability, and enable the selection of a single longitudinal mode, a tunable laser can be designed as a laser cavity architecture with multiple cascaded filters. For example, the laser cavity architecture may include a gain section in which the laser is generated, and front and rear mirrors with grating patterns that are periodically spatially modulated in amplitude and / or phase to produce a comb-like multi-peak reflectivity spectrum. The front and rear mirrors may have unequal repetition periods (lengths) to achieve a vernier effect (e.g., the current applied to gratings with different periods can be varied to adjust the effective grating spacing, thereby achieving wide-range wavelength tuning with high precision). For example, the rear mirror can be tuned (e.g., to a shorter wavelength) to achieve large discrete jumps to adjacent sets of mirror peaks via a vernier effect; the front mirror can be tuned to achieve spectral jumps to different supermodes at much longer wavelengths; or the two mirrors can be tuned synchronously to tune any two mirror peaks together (on the same supermode) to select a longitudinal mode between the wavelengths of the untuned mirror peaks. In this way, any mode can be selected by an appropriate combination of differential and synchronous tuning, and with additional phase tuning, any arbitrary target wavelength can be achieved.

[0017] While the vernier effect enables enhanced (full-band) tuning range, it is practically limited because the spectral comb spacing difference between the front and rear mirrors must be sufficiently large compared to the spectral width of each mirror peak to attenuate spectral overlap of side supermodes, otherwise reducing the SMSR. Furthermore, the comb mirror spectrum produces repetitive modes, where two pairs of peaks are simultaneously aligned at opposite ends of the tuning range, with the associated repetitive mode spacing serving as an upper limit with respect to the tuning range. To eliminate the repetitive mode effect, a finite number of mirror peaks is desired such that the peak spacing multiplied by the number of peaks is less than the repetitive mode spacing. However, this is practically impossible for a sampling grating because a rectangular sampling function produces a comb spectrum modulated by a sinusoidal spectral envelope. On the other hand, a phase grating can be used instead of a sampling grating to provide greater freedom in designing the grating's reflection spectrum, where optimizing the number and placement of phase shifts can approximately reduce unwanted peaks by more than a factor of the spectral envelope. However, when applied to vernier-tuned lasers, the phase grating method has several drawbacks, including the challenge of independently controlling the peak reflectivity and spectral width in the front and rear mirrors, as well as eliminating peaks outside the desired tuning range.

[0018] In some embodiments, as described herein, wavelength-tunable semiconductor lasers have a design to eliminate repetitive modes, wherein a first pair of front and rear mirror peaks and a second pair of front and rear mirror peaks at opposite ends of the tuning range are simultaneously aligned. For example, in some aspects, the wavelength-tunable semiconductor laser includes a gain section, a front mirror comprising a first integer number of repetitive units, and a rear mirror comprising a second integer number of repetitive units. In some embodiments, the first integer number of repetitive units includes a first set of phase grating elements (where each phase grating element includes an etched region paired with an unetched region) having a first plurality of π-phase shifts distributed within the first set of phase grating elements, and the second integer number of repetitive units includes a second set of phase grating elements having a second plurality of π-phase shifts distributed within the second set of phase grating elements. In some implementations, the first plurality of π-phase shifts differ from the second plurality of π-phase shifts by one phase shift per repetitive unit (e.g., where the phase shifts are implemented using etched regions or unetched regions having a length different from that of the phase gratings in the absence of phase shifts). Furthermore, in some embodiments, the first integer number of repeating units differs from the second integer number of repeating units. In some embodiments, the phase grating element of the front mirror is configured to generate a first comb-shaped reflection spectrum and a second comb-shaped reflection spectrum, each comb-shaped reflection spectrum comprising multiple reflection peaks substantially within the target spectral envelope (e.g., an approximate top-cap spectral envelope with some permissible structure below the target spectral envelope to compensate for the gain envelope). Additionally, any peaks outside the target spectral envelope are reduced by a desired percentage or ratio relative to peaks within the target spectral envelope.

[0019] In this way, by eliminating repetitive patterns in which mirror peaks at opposite ends of the tuning range are simultaneously aligned, some embodiments described herein enable improved manufacturing tolerances and spectral purity for wavelength-tunable semiconductor lasers. For example, some embodiments described herein eliminate repetitive patterns that could otherwise cause the gain spectrum to define the actual wavelength range, making channel coverage highly sensitive to the material composition of the active (gain) region. Furthermore, by providing a target spectral envelope approximating a top-hat structure, some embodiments described herein generate mirror reflectivity peaks outside the desired tuning band that drop to near zero on two orders. Moreover, some embodiments described herein, without altering the envelope associated with the reflectivity peaks, enable the adjustment of mirror reflectivity peaks by omitting one or more gratings from the repetitive units, allowing the front and rear mirrors to be fabricated in a single etching step, and permitting the use of a mixed number of peaks while clearing any unwanted peaks on the short and long sides of the tuning range, thereby providing a substantial improvement in sensitivity to the gain spectrum.

[0020] Figure 1 The illustration shows an example tunable semiconductor laser 100. (Example:) Figure 1 As shown, the tunable semiconductor laser 100 includes a gain region 102 disposed between a first mirror 104 and a second mirror 106, and a filter 108 disposed between the gain region 102 and the second mirror 106. The tunable semiconductor laser 100 is formed by the gain region 102, which is electrically injected and patterned as an optical waveguide with cleaved and / or coated facets. For example, laser light generated within the gain region 102 is reflected by the second mirror 106, wherein a laser cavity (e.g., the region between the first mirror 104 and the second mirror 106) provides feedback that allows for amplification and lasing of a specific wavelength. Furthermore, to make the laser 100 tunable, the resonance or gain peak within the cavity is adjustable (e.g., by changing the carrier density in different portions via electrical and thermal tuning to modify the refractive index of the semiconductor waveguide, and thus change the effective cavity optical length).

[0021] However, monolithically integrated tunable lasers (e.g., for telecommunications or other suitable applications) are typically designed to have a tuning range that satisfies a full communication bandwidth (e.g., greater than 40 nm in the C-band near 1550 nm wavelengths, or greater than 48 nm for extended bands) with an SMSR (e.g., greater than 40 dB). In semiconductor lasers formed solely by the electrically injected gain region, such as Figure 1 The laser shown has a gain bandwidth that prevents semiconductor lasers from having the ability to lase at a single frequency (or a single longitudinal mode). Instead, the laser cavity supports multiple longitudinal modes, each with a corresponding resonant wavelength and frequency, with the cyclic light waves constructively superimposed for each round trip. In other words, the round-trip phase propagating within the laser cavity is equal to an integer multiple of 2π. To select a single longitudinal (cavity) mode with high SMSR, the laser cavity incorporates narrow passband filtering elements, such as... Figure 1 As shown, filter 108 is disposed between gain region 102 and second reflector 106.

[0022] For example, when the laser generated within the gain region 102 circulates within the laser cavity between the first reflector 104 and the second reflector 106, any given longitudinal mode is associated with the lasing threshold oscillation condition or the unit round-trip gain given by the following formula: in, , It is the reflectivity of a complex mirror. It is the transmittance of the round-trip filter. It is the wavelength in free space. It is the threshold mode gain. It is waveguide loss. ,and This is the effective refractive index of the waveguide. The wavelength corresponds to an integer multiple of the 2π round-trip phase, and the threshold mode gain corresponds to the unit round-trip gain amplitude. Filter 108 has high transmittance (and thus low threshold gain) in the desired lasing mode and must be narrow enough to select a single cavity mode while exhibiting reduced transmittance (or high loss) in all other longitudinal modes. As a result, the precise lasing wavelength is determined based on the cavity (longitudinal) mode, and for high SMSR, the threshold gain of the undesired side modes is increased sufficiently. For example, as shown in example curve 110, the laser cavity generates many longitudinal modes with different resonant wavelengths within the gain spectrum, and filter 108 is tuned to select a single longitudinal mode with high SMSR (e.g., depending on the physical implementation, filter 108 can be thermally, mechanically, and / or electronically tuned).

[0023] As mentioned above, providing Figure 1 As an example. Other examples may differ from those regarding... Figure 1 The example described.

[0024] Figure 2 An example distributed Bragg reflector (DBR) laser 200 and an example 220 relating to the tuning range of the DBR laser 200 are illustrated. Specifically, as described herein, the DBR laser 200 employs a grating 210 etched into a passive semiconductor waveguide to reflect only narrowband wavelengths, thus providing both filtering and light reflection capabilities in a single element. For example, Figure 2 The illustrated DBR laser 200 includes a rear mirror 206 implemented as a DBR (e.g., via grating 210) to combine filtering and reflection functions, and a front mirror 204, which is an etched surface with a fixed broadband reflectivity (e.g., enhanced with a suitable dielectric coating to adjust the reflectivity for optimal output coupling). The DBR (or rear mirror) 206 has tuning electrodes 212, which can be used to adjust the refractive index of one or more layers of the semiconductor waveguide of the DBR 206 by carrier injection or Joule heating.

[0025] In this way, the effective refractive index of the waveguide of the patterned grating 210 can be tuned. The pitch of the grating 210 is selected to position the optical reflectivity band close to one end of the desired tuning range, allowing the optical reflectivity band to be tuned to shorter wavelengths (e.g., if carrier injection is used) or, in a controllable amount, to longer wavelengths (e.g., if thermal tuning is used). In addition to the DBR 206, the DBR laser 200 includes a gain section 202 controlled by an additional electrode 212 for injecting current into a suitably designed active region to provide optical gain. The DBR laser 200 includes a phase section 208 with electrode 212 providing control terminals for fine-tuning the longitudinal mode spectrum. However, carrier injection or thermal tuning cannot produce a sufficient refractive index shift to achieve full-band tunability (e.g., single-sided DBR lasers typically achieve a tuning range of 12–15 nm optimally, as shown in Example 220).

[0026] As mentioned above, providing Figure 2 As an example. Other examples may differ from those regarding... Figure 2 The example described.

[0027] Figure 3 The illustration shows an example sampled-grating DBR (SG-DBR) laser 300 and an example 320 indicating the reflectivity of the front and rear mirrors and laser emission at various wavelengths. For example, a laser cavity architecture with multiple cascaded filters can use the vernier effect to overcome refractive index tuning limitations and achieve full-band tunability while still selecting for a single longitudinal mode. The SG-DBR laser 300 has a four-part laser cavity architecture using the vernier effect, wherein the laser cavity architecture includes a gain region 302 controlled by electrodes for current injection to provide optical gain, a front DBR mirror 304 and a rear DBR mirror 306 with etched gratings 310, and a phase region 308 with electrodes providing control terminals to fine-tune the longitudinal mode spectrum. However, the gratings 310 in the front DBR mirror 304 and the rear DBR mirror 306 have patterns that are periodically and spatially modulated in amplitude and / or phase to produce a comb-like multi-peak reflectivity spectrum. Furthermore, the repetition periods (or lengths) in the front reflector 304 and the rear reflector 306 are not equal to achieve the vernier effect.

[0028] In the SG-DBR laser 300, the grating 310 follows a periodically sampled (or conversely, blanked) grating function. For example, as Figure 3As shown, the front DBR mirror 304 and the rear DBR mirror 306 are each formed by short pulses of a grating 310 spaced apart by longer blank areas (e.g., like a spatially uniform grating 310 multiplied by a periodic rectangular sampling function with a fixed repetition period, hence the term "sampling grating"). The untuned spectra of the front mirror 304 and the rear mirror 306 are shown in Example 320-1, where only two intermediate peaks are aligned due to unequal comb spacing. Furthermore, the combination of aligned mirror peaks is called a supermode. In Example 320-1, the combined (multiplied) filter spectrum for the supermode is selected as a single longitudinal mode corresponding to the lasing spectrum labeled as reference numeral 322-1. By tuning the rear mirror 306 (e.g., to a shorter wavelength), large discrete jumps to adjacent sets of mirror peaks (adjacent supermodes) are achieved through a vernier effect, as shown in the lasing spectrum labeled as reference numeral 322-2 in Example 320-2. Similarly, tuning the front mirror 304 individually may allow jumping to the lasing spectrum labeled as reference numeral 322-3 in Example 320-3 on different supermodes at much longer wavelengths. Finally, tuning both the front mirror 304 and the rear mirror 306 synchronously allows any two mirror peaks to be tuned together (on the same supermode) to select a longitudinal mode between the wavelengths of the untuned mirror peaks, as shown in the lasing spectrum labeled as reference numeral 322-4 in Example 320-4. In this way, any mode can be selected by an appropriate combination of differential and synchronous tuning, and any arbitrary target wavelength can be achieved by adding phase tuning.

[0029] As mentioned above, providing Figure 3 As an example. Other examples may differ from those regarding... Figure 3 The example described.

[0030] Figure 4 The illustration shows an example 400 related to the tuning range, which can be determined based on a laser cavity architecture using the vernier effect (such as...). Figure 3 The vernier effect is determined by the difference between the mirror peak spacing and the spacing between the front and rear mirrors in the SG-DBR laser 300 shown. Although the vernier effect enables an enhanced (full-band) tuning range, it is practically limited. For example, the difference in spectral comb spacing between the front and rear mirrors can be expressed as... ,in It is the spacing between the peaks of the front reflector, and This refers to the spacing between the peaks of the rear mirror. Typically, the difference in spectral comb spacing between the front and rear mirrors must be sufficiently large compared to the spectral width of individual mirror peaks to attenuate spectral overlap of the side supermodes, otherwise it will reduce the SMSR. Additionally, the comb mirror spectra typically produce repeating modes, whereby the first pair of peaks is aligned at one end of the tuning range, and the second pair of peaks is aligned at the opposite end of the tuning range, with associated repeating mode spacing (RMS), as shown in Example 400.

[0031] As stated in this article, equals The RMS value is the upper limit of the tuning range for a laser cavity architecture using the vernier effect. Because the two competing supermodes are spectrally far apart, the combination of the gain spectrum and the mirror spectrum envelope determines whether a shorter or longer wavelength has the lowest threshold and becomes the dominant laser supermode. In some cases, the gain spectrum shape exactly cancels out the mirror peak reflectivity, neither supermode is dominant, and the laser exhibits poor SMSR, thus reducing the effective tuning range. The net effect is that vernier tuning places strict manufacturing tolerances on the active region material composition and layer thickness placement to achieve the repeatability required in the gain spectrum. If the gain spectrum is unintentionally shifted due to manufacturing variations, the resulting tuning range may not meet channel planning constraints. Alternatively, the laser can be overdesigned within the tuning range by 20% or more to accommodate variations in the gain spectrum, at the cost of additional tuning power dissipation or other design compromises.

[0032] As mentioned above, providing Figure 4 As an example. Other examples can be related to... Figure 4 The descriptions are different.

[0033] Figure 5Examples 500 and 520 are illustrated in relation to grating modulation functions for an ideal comb reflector. To eliminate the aforementioned repetitive mode effect, a finite number of mirror peaks is desired such that the peak spacing multiplied by the number of peaks is less than the repetitive mode spacing, which is practically impossible to achieve with a sampling grating (e.g., because a rectangular sampling function produces a comb spectrum modulated by a sinusoidal spectral envelope). On the other hand, phase gratings provide a method for engineering the reflection spectrum. Therefore, some embodiments described herein relate to wavelength-tunable semiconductor lasers using phase gratings instead of sampling gratings, where the phase grating provides greater freedom in designing the reflection spectrum of the grating. The phase grating begins with a uniform grating that undergoes polarity reversal (or π-phase shift) at multiple locations within a repetitive unit, which is part of the overall grating. For example, as described herein, a series of alternating masks (M) and intervals (S) represents a repeating unit, where each mask is an unetched region and each interval is an etched region, and phase shifting can be achieved in a phase grating with masks or intervals whose length is twice that of a mask or interval in a phase grating without phase shifting. The entire grating comprises multiple repeating units cascaded together (e.g., on the order of approximately 5 to 15 repeating units). Each repeating unit has a length called the repeat length.

[0034] In some implementations, the number and location of phase shifts can be optimized to approximate the top-cap spectral envelope of peaks with equal reflectance, as shown in Example 500. Alternatively, as shown in Example 520, a fixed number of peaks with a relatively uniform spectral envelope (e.g., the seven in Example 520) can be implemented using a phase-shifting grating, where undesired peaks are reduced by more than a factor of two. For example, a phase-shifting grating is a uniform grating that introduces a π-phase shift at appropriate locations, and the design of the phase-shifting grating can be optimized using numerical techniques. Furthermore, suitable techniques can be used to optimize the enhanced reflectance of outer peaks and the reduced reflectance of mid-band peaks to compensate for the gain spectral shape.

[0035] However, the phase grating method has drawbacks when applied to vernier-tuned lasers. For example, independent control of peak reflectivity and spectral width in the front and rear mirrors is challenging unless the grating intensity (grating ripple depth) is controlled independently in both DBRs. Specifically, the front mirror has a lower (less than half) peak reflectivity than the rear mirror to achieve high front facet slope efficiency, which can be addressed by combining a sampling grating front mirror with an upper-layer structure or a phase grating rear mirror. Additionally, peaks outside the required tuning range should be close to zero.

[0036] As mentioned above, providing Figure 5 As an example. Other examples may differ from those regarding... Figure 5 The example described.

[0037] Figure 6A An example of a vernier-tuned DBR laser 600 according to some embodiments described herein is illustrated. This vernier-tuned DBR laser 600 is a wavelength-tunable semiconductor laser based on a phase grating, featuring improved supermode suppression manufacturing tolerances. Additionally, Figure 6B The illustration shows an example tuning range 620 for a vernier-tuned DBR laser 600. Figure 6C The illustration shows example reflectances of 630 and 635 within and outside the target's spectral envelope. Figure 6D The illustration shows an example comb reflection spectrum 640 generated by a vernier-tuned DBR laser 600, and Figure 6E An example wavelength diagram 660 of a vernier-tuned DBR laser 600 is illustrated. More specifically, to improve manufacturing tolerances and spectral purity, the vernier-tuned DBR laser 600 is designed to eliminate the aforementioned repetitive modes, wherein a first pair of front and rear mirror peaks are aligned at a first end of the tuning range, and a second pair of front and rear mirror peaks are aligned at opposite ends of the tuning range. When repetitive modes are present, the gain spectrum determines the actual wavelength range, making channel coverage highly sensitive to the material composition of the active region.

[0038] Therefore, a vernier-tuned DBR laser 600 is implemented to eliminate repetitive modes having a wavelength-tunable semiconductor laser structure including a gain portion 602, a front mirror 604 having a first integer number of repetition units 610, and a rear mirror 606 having a second integer number of repetition units 610. In some embodiments, the first integer number of repetition units 610 may include phase grating elements having a first plurality of π-phase shifts distributed among phase grating elements, and the second integer number of repetition units 610 may include phase grating elements having a second plurality of π-phase shifts distributed among phase grating elements. As described herein, the term "phase grating element" refers to a mask (corresponding to an unetched area or area with unetched material) paired with a spacing (corresponding to an etched area or area with etched material).

[0039] For example, in Figure 6A In the accompanying drawings, reference numerals 610-1, 610-2, and 610-3 each depict a repeating unit 610, which includes... N A phase grating element, each phase grating element including S denoted as S i The intervals are represented as M. iThe mask. Furthermore, the phase grating element with phase shift is implemented using a mask or spacing, the length of which is twice the length of the mask or spacing of the phase grating element without phase shift. For example, in the repeating unit 610 depicted by reference numerals 610-1, 610-2, and 610-3, in the... N There is a phase shift at each phase grating, where the interval S N The length of the phase offset is greater than the length of the interval in the first ten phase gratings. For example, as described herein, the pitch is defined by the combined length of a mask and an interval, and the phase offset is equal to half the pitch (e.g., when the mask and interval have the same length, the length of the interval with the phase offset is twice the length of the other intervals, or the mask:interval ratio is equal to 1). Furthermore, additional phase offsets may be present at other locations within a given repeating unit (e.g., in a repeating unit 610 with 209 phase gratings, six phase offsets may be distributed at different locations among the 209 phase gratings). In some embodiments, the first plurality of π phase offsets and the second plurality of π phase offsets differ by one phase offset per repeating unit. As described herein, a π phase offset is equivalent to or similar to a 180-degree phase offset, where the wave is reversed such that the position and direction of the wave are reversed at the point where the phase offset occurs.

[0040] In some implementations, such as Figure 6A As shown, the wavelength-tunable semiconductor laser structure may further include a phase portion 608 with control terminals, wherein current can be injected to finely tune the longitudinal mode spectrum. In some embodiments, the integer number of repeating units 610 in the front mirror 604 differs from the integer number of repeating units 610 in the rear mirror 606. For example, adding more repeating units 610 typically increases the reflectivity of each peak and narrows the spectral width of each peak while maintaining a similar relative envelope. To direct most of the laser emission away from the front facet, some embodiments may include fewer repeating units 610 in the front mirror 604 and more repeating units 610 in the rear mirror 606. However, there is no specific limitation on the difference between the integer number of repeating units 610 in the front mirror 604 and the integer number of repeating units 610 in the rear mirror 606, except that a sufficient number of repeating units 610 are used as a sufficiently narrow spectral filter function to generate the product of the front mirror 604 and the rear mirror 606 to select a single longitudinal mode. In some implementations, a mirror with a larger peak spacing and therefore a shorter repeat length may have one less phase shift per repeating unit compared to a mirror with a smaller peak spacing or a longer repeat length. In some implementations, the front mirror 604 can be modified by removing a subset of the phase grating elements (e.g., replacing etched areas or gaps with unetched material).

[0041] For example, refer to Figure 6A Reference numerals 610-1, 610-2, and 610-3 provide example grating definitions (or configurations) for phase grating elements in repeating unit 610, wherein each phase grating element includes a pair of masks spaced to correspond to etched regions and to correspond to unetched regions, wherein the masks and spacing with phase offsets are double the length. For example, in Figure 6A In the middle, marked as M i and S i The components form the first i A phase grating element (e.g., elements labeled M1 and S1 form a first phase grating element, elements labeled M2 and S2 form a second phase grating element, and so on). In Figure 6A In the figure, reference numeral 610-1 depicts a repeating unit 610 with a grating definition, wherein all of the grating is maintained. Reference numeral 610-2 depicts a repeating unit 610 with grating clarity, wherein 20% of the grating is removed (e.g., every fifth grating is removed, as shown by the circled unetched areas M5 and M6 respectively). 10 Replace etched areas S5 and S 10 As shown in the figure), and reference numerals 610-3 depict repeating units 610 with raster clarity, wherein 33.3% of the raster is removed (e.g., every third raster is removed). Thus, as described herein, removing the raster (or replacing the etched area with unetched material) results in three consecutive unetched areas (e.g., at the location where the raster is removed, plus two unetched areas adjacent to the removed raster).

[0042] As described herein, in a vernier-tuned DBR laser 600, one mirror (e.g., a mirror with a larger comb spacing, typically the front mirror 604) has a mirror peak that is one size smaller than that of the other mirror, without reducing the tuning range. Figure 6B As shown in Example 620, the tuning range of the tunable laser includes nine mirror peaks for one mirror (e.g., the rear mirror 606 in the illustrated example) and eight mirror peaks for another mirror (e.g., the front mirror 604 in the illustrated example). To reduce the number of mirror peaks and thus mitigate repetitive mode effects, the front mirror 604 is designed to have one less phase shift than the rear mirror 606 (and therefore, the total number of phase shifts for each repetitive unit 610 in the front mirror 604 is odd, resulting in an even number of mirror peaks). In contrast, conventional designs are limited to an even number of shifts (and therefore an odd number of mirror peaks), such that the mid-peak wavelength substantially matches that of a conventional uniform grating.

[0043] In some implementations, the vernier-tuned DBR laser 600 uses a mirror design with a spectral envelope in which the mirror reflectivity peak outside the required tuning band drops to near zero, or at least five times (e.g., more than 10 dB less than within the band), by two orders of magnitude. For example, the envelope could be similar to... Figure 5 The top-cap envelope shown in Example 500 can be implemented using an odd number of offsets per repeating unit (e.g., as shown in the spectrum of the front mirror 604 in Example 620). In some embodiments, the spectral envelope may include some structure beneath the envelope to compensate for the gain envelope. For example, according to some embodiments, the phase grating element of the front mirror 604 may be configured to generate a first comb-shaped reflectance spectrum having a first plurality of reflectance peaks substantially within the target spectral envelope. In some embodiments, the phase grating element of the rear mirror 606 may be configured to generate a second comb-shaped reflectance spectrum having a second plurality of reflectance peaks substantially within the target spectral envelope. In some embodiments, the first plurality of reflectance peaks have one fewer peak than the second plurality of reflectance peaks (e.g., in Example 620, the rear mirror 606 generates 9 reflectance peaks, and the front mirror generates 8 reflectance peaks). In some implementations, peaks outside the target envelope can be reduced by a desired percentage relative to peaks within the envelope, for example, by at least five times (e.g., as shown in Example 620, the height of the undesired peaks generated by the front mirror 604 and the rear mirror 606 relative to the heights of the peaks of the front mirror 604 and the rear mirror 606 falling within the laser tuning range). In some implementations, the peak structure within the envelope (e.g., within the laser tuning range) can compensate for the material-based spectrum of the gain region 602.

[0044] For example, Figure 6CThe illustration shows example reflectance 630 within and outside the target spectral envelope for front mirror 604 and rear mirror 606, wherein, relative to the peaks within the target band, two peaks located outside and immediately adjacent to the low side of the target band, and two peaks located outside and immediately adjacent to the high side of the target band, are reduced by at least five times. For example, example reflectance 630 within the target band in the range of 1520–1575 nm is shown for front mirror 604 and rear mirror 606, and example reflectance 635 within the target band is shown in more detail only for front mirror 604. In example reflectance 635, two peaks outside and immediately adjacent to the low side of the target band, and two peaks outside and immediately adjacent to the high side of the target band, are enclosed within a dashed circle. As shown in the figure, the peak reflectivity of the front mirror 604 is approximately 30% within the target wavelength band, the reflectivity of the first pair of peaks outside and immediately adjacent to the target wavelength band is approximately 2.4%, and the reflectivity of the second pair of peaks outside and immediately adjacent to the target wavelength band is approximately 4.4%. Therefore, the first and second pairs of peaks outside the target wavelength band readily satisfy the five-fold condition. Although the third pair of peaks outside the target wavelength band has higher reflectivity, this higher reflectivity is not significant because the gain spectrum is not broad enough to allow the third pair of peaks (or other peaks far from the target wavelength band) to compete with the peaks within the target wavelength band. Therefore, as described herein, the first pair of peaks outside and immediately adjacent to the target wavelength band is at least five times smaller than the peaks within the target wavelength band, and the second pair of peaks outside and immediately adjacent to the target wavelength band is preferably also at least five times smaller than the peaks within the target wavelength band.

[0045] In some embodiments, because the reflectivity peaks of the mirrors can be tuned by eliminating a portion of the grating without altering the envelope of the reflectivity peaks, the front mirror 604 and rear mirror 606 of the vernier-tuned DBR laser 600 can be fabricated in a single etching step. In some embodiments, the vernier-tuned DBR laser 600 may be designed to simultaneously employ a mixed number of peaks (e.g., eight and nine) and to zero out two unwanted peaks on both the short and long sides of the tuning range, which can substantially improve sensitivity to the gain spectrum.

[0046] According to some embodiments, the vernier-tuned DBR laser 600 can use a grating design to mitigate or eliminate grating comb teeth. In some embodiments, the desired reflection spectrum can be designed by iteratively modeling the reflection of the cascaded group for the repeating units 610. Adjustable parameters (for a given etch depth) can include the number of grating elements (masks and spacing), the number of repeating units 610, the number of phase shifts, the location of the phase shifts, the location of the removed grating elements, and / or the number of removed grating elements. For example, the grating (or mirror) can have a pitch in the range of 0.2 to 0.3 micrometers (μm), wherein the pitch is determined by dividing the wavelength by twice the waveguide refractive index, where the pitch is the length of a mask plus a spacing. The reflection spectrum of the grating can be tuned by introducing phase shifts into the grating. For example, a phase shift can be introduced by doubling the length of the spacing or mask in the grating. Each grating can include multiple phase shifts in the range of 1 to 20. The total length of the repeating units can range from 1 to 100 μm and can be determined by multiplying the pitch by the number of masks / spacers plus half the number of phase shifts, and the number of repeating units can range from 2 to 30. Etching depth (e.g., for spacers) is a parameter describing the depth to which the spacers are etched into the waveguide, and grating intensity (how much light is reflected per unit length) is another design parameter. For example, the grating intensity parameter can be adjusted by regulating the grating etching depth. The front mirror 604 and the rear mirror 606 can have different reflectivities and different grating intensities, resulting in different etching depths for the front mirror 604 and the rear mirror 606. Alternatively, reflectivity can be controlled by removing a portion of the grating on one mirror (e.g., the front mirror 604), in which case the front mirror 604 and the rear mirror 606 can have the same etching depth.

[0047] In some embodiments, the number of repeating units 610 and the number of removed grating elements can determine the total reflectivity of the mirror and the width of the reflectivity peaks. As described elsewhere herein, the front mirror 604 can be designed to have less reflectivity than the rear mirror 606 in order to couple light out of the laser cavity. The number of dominant comb peaks in the spectrum can be determined by the number of phase shifts. The front mirror 604 and the rear mirror 606 are designed with different numbers of phase shifts, and therefore different numbers of dominant comb peaks. The number of peaks in each comb and the peak spacing are determined by the desired tuning range of the laser 600. In some embodiments, the number and position of phase shifts determine the overall spectral shape. For example, the number and position of phase shifts can determine how quickly the reflection intensity drops outside the desired range, such as... Figure 6D Example 640 in the image is shown through the lower peaks on the left and right sides. These parameters can also be adjusted for the overall profile shape of the spectrum (e.g., as shown in the image). Figure 5(As shown in Example 500). Also, as... Figure 6D As shown in Example 640, the central portion of the comb spectrum falls in the middle, which can be intentionally tuned to compensate for peaks in the gain spectrum of the laser material (e.g., as shown in Example 640). Figure 1 The gain spectrum curve in example curve 110 is shown. The number of comb teeth removed determines the reflectance of the entire spectral peak (e.g., as shown in example 640, where the reflectance of the mirror peak varies with grating removal, where the highest peak reflectance is achieved without grating removal and the lowest peak reflectance is achieved with 33.3% of the grating removed). In some embodiments, the grating can be removed periodically (e.g., removing every...). n (A grating). Alternatively, the grating can be removed randomly, which can introduce more loss to the grating compared to periodic removal.

[0048] refer to Figure 6E Example 660 illustrates the wavelength plot for a vernier-tuned DBR laser 600 in a design where the front mirror 604 has eight peaks and the rear mirror 606 has nine peaks. For example, unwanted peaks on both the short and long sides of the tuning range are more than 3 dB lower than peaks in the middle of the tuning range, and unwanted supermodes shifted to the upper left or lower right of the wavelength plot do not occupy the dominant supermode region. In this way, the sensitivity to the gain spectrum is significantly improved. The front mirror 604, with a larger peak spacing and therefore a shorter repetition length (e.g., a total of five phase shifts per repetition unit 610), has one less phase shift per repetition unit 610 than the rear mirror 606, which has a smaller peak spacing or a longer repetition length (e.g., a total of six phase shifts per repetition unit).

[0049] In some embodiments, the vernier-tuned DBR laser 600 may include substantially similar different repeating units 610. For example, a DBR grating may include a plurality of repeating units 610, wherein each repeating unit 610 includes a plurality of phase grating elements and a plurality of π phase shifts. In some embodiments, the number of phase grating elements in each repeating unit of the plurality of repeating units 610 is substantially the same (but not necessarily exactly the same). Additionally or alternatively, the number and position of the plurality of π phase shifts are substantially the same (but not necessarily exactly the same) in each repeating unit of the plurality of repeating units 610. In some embodiments, a pair of DBRs (e.g., front mirror 604 and rear mirror 606) may differ in phase shift per repeating unit 610. For example, in a pair of DBR gratings, each DBR in the pair has a different number of phase shifts in each repeating unit 610 (e.g., thus producing a different number of peaks from each grating). In some embodiments, a DBR pair with a smaller repeat length has one less phase shift per repeating unit 610 (e.g., thus producing a mirror with a smaller number of peaks and a larger peak spacing). In some embodiments, a wavelength-tunable semiconductor laser may include a gain portion 602, a phase portion 608, and a pair of DBR gratings 604, 606 as described herein.

[0050] In some implementations, as described herein, a phase grating element may include a mask paired with a spacing, and the phase shift is generated by inserting additional (unpaired) masks or additional (unpaired) spacings between the grating elements. The grating pitch may correspond to the length of the phase grating element such that the phase grating element occupies two half-pitches, and the phase shift is inserted into additional half-pitches. Therefore, the number of grating half-pitches per repeating length is... ,in It is the number of phase grating elements and It is the number of phase shifts. Therefore, an even number of phase shifts per repeat length results in an even number of grating half-pitches, and an odd number of phase shifts results in an odd number of grating half-pitches.

[0051] In some implementations, the phase shift can correspond to a polarity reversal, such as MSMS becoming SMSM or vice versa, instead of inserting an additional single S or M into the (MSMS) stream. In this way, the phase grating can be constructed with an odd number of phase shifts per repeating unit, resulting in an even number of half-pitches. In this case, each successive repeating unit 610 can be opposite rather than strictly identical. Similarly, repeating units 610 can be constructed using the same technique with an even number of phase shifts and an odd number of half-pitches. Thus, the phase grating can generally exhibit polarity reversal between successive repeating units 610, where the original repeating unit following a reversed polarity version of repeating unit 610 can be considered a "super-repeat unit" and / or two substantially similar and dissimilar repeating units 610.

[0052] As mentioned above, providing Figures 6A-6E As an example. Other examples may differ from those regarding... Figures 6A-6E The example described.

[0053] Figure 7 The illustration shows an example process for operating a wavelength-tunable laser. Figure 7 One or more process blocks are executed by a wavelength-tunable semiconductor laser (e.g., a vernier-tuned DBR laser 600).

[0054] like Figure 7 As shown, process 700 includes generating light (block 710) from the gain portion of a wavelength-tunable semiconductor laser. For example, vernier-tuned DBR laser 600 may include a gain portion 602 configured to generate light, as described above.

[0055] For example Figure 7 As shown, process 700 includes reflecting light through a first mirror comprising a first integer number of repeating units, wherein each repeating unit comprises a plurality of phase grating elements having a first plurality of π phase shifts distributed among the plurality of phase grating elements (block 720). For example, vernier-tuned DBR laser 600 may include a rear mirror 606 configured to reflect light, wherein the rear mirror 606 may include a first integer number of repeating units 610, each repeating unit comprising a plurality of phase grating elements, wherein, as described above, the first plurality of π phase shifts are distributed among the plurality of phase grating elements.

[0056] For example Figure 7As shown, process 700 includes a first portion of output light and a second portion of light reflected by a second mirror comprising a second integer number of repetition units, wherein each of the second integer number of repetition units comprises a plurality of phase grating elements having a second plurality of π-phase shifts distributed among the plurality of phase grating elements, wherein the first plurality of π-phase shifts and the second plurality of π-phase shifts differ by one phase shift per repetition unit (block 730). For example, vernier-tuned DBR laser 600 may include a front mirror 604 configured to output the first portion of light and reflect the second portion of light, wherein the front mirror 604 comprises a second integer number of repetition units 610, each repetition unit comprising a plurality of phase grating elements having a second plurality of π-phase shifts distributed among the plurality of phase grating elements as described above. In some aspects, the first plurality of π-phase shifts and the second plurality of π-phase shifts differ by one phase shift per repetition unit.

[0057] Process 700 may include additional aspects, such as any single aspect or any combination of aspects of one or more other processes described below and / or elsewhere herein.

[0058] In the first aspect, process 700 includes adjusting the optical length of the optical cavity of the vernier-tuned DBR laser 600 by means of a cavity optical length adjustment portion (e.g., phase portion 608) arranged between the gain portion 602 and the front mirror 604 or the rear mirror 606, so as to adjust the refractive index of the optical cavity and modulate the longitudinal mode spectrum of the light generated by the gain portion 602.

[0059] In the second aspect, either alone or in combination with the first aspect, the first integer number of repeating units 610 is different from the second integer number of repeating units 610.

[0060] In the third aspect, either alone or in combination with one or more of the first and second aspects, the number of the first plurality of π phase offsets is one less than the number of the second plurality of π phase offsets.

[0061] In the fourth aspect, either alone or in combination with one or more of the first to third aspects, each of the first integer number of repeating units 610 and the second integer number of repeating units 610 contains a plurality of phase grating elements that include unetched regions paired with etched regions.

[0062] In the fifth aspect, either alone or in combination with one or more of the first to fourth aspects, the first integer number of repeating units 610 also includes a plurality of unetched regions.

[0063] In the sixth aspect, either alone or in combination with one or more of the first to fifth aspects, each of the plurality of phase grating elements in the first integer number of repeating units 610 and the plurality of phase grating elements in the second integer number of repeating units 610 includes a first subset of phase grating elements having a first length and a second subset of phase grating elements having a second length longer than the first length, and the first plurality of π phase offsets and the second plurality of π phase offsets are each distributed in the second subset of phase grating elements having the second length.

[0064] In the seventh aspect, either alone or in combination with one or more of the first to sixth aspects, a first integer number of repeating units 610 in the front reflector 604 are configured to generate a first comb-shaped reflection spectrum having a first plurality of reflection peaks within the target spectral envelope, and a second integer number of repeating units 610 in the rear reflector 606 are configured to generate a second comb-shaped reflection spectrum having a second plurality of reflection peaks within the target spectral envelope, wherein the first plurality of reflection peaks has one less reflection peak than the second plurality of reflection peaks.

[0065] In the eighth aspect, either alone or in combination with one or more of the first to seventh aspects, the first comb reflectance spectrum and the second comb reflectance spectrum each further include one or more reflectance peaks outside the target spectral envelope, which are at least five times smaller than the first plurality of reflectance peaks and the second plurality of reflectance peaks inside the target spectral envelope.

[0066] In the ninth aspect, either alone or in combination with one or more of the first to eighth aspects, the first plurality of reflection peaks and the second plurality of reflection peaks within the target spectral envelope have a material-based spectral shape of the compensated gain portion 602.

[0067] although Figure 7 An example block of process 700 is shown, but in some implementations, process 700 includes... Figure 7 The blocks described herein are compared to additional blocks, fewer blocks, different blocks, or blocks with different arrangements. Alternatively, two or more blocks of process 700 can be executed in parallel.

[0068] The foregoing disclosure provides illustrations and descriptions, but is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Modifications and variations can be made based on the foregoing disclosure, or modifications and variations can be derived from practice of the embodiments. Furthermore, any embodiments described herein can be combined unless the foregoing disclosure expressly provides reasons why one or more embodiments may not be combined.

[0069] As used in this article, depending on the context, satisfying the threshold can refer to a value that is greater than the threshold, greater than or equal to the threshold, less than the threshold, less than or equal to the threshold, equal to the threshold, not equal to the threshold, etc.

[0070] Even if a particular combination of features is recited in the claims and / or disclosed in the specification, these combinations are not intended to limit the disclosure of various embodiments. In fact, many of these features can be combined in ways not specifically recited in the claims and / or disclosed in the specification. Although each dependent claim listed below may directly depend on only one claim, the disclosure of various embodiments includes each dependent claim in combination with every other claim in the claim set. As used herein, the phrase “at least one” in the list of items refers to any combination of those items, including a single member. As an example, “at least one of: a, b, or c” is intended to cover a, b, c, ab, ac, bc, and abc, as well as any combination of multiple identical items.

[0071] When a component or one or more components (e.g., a laser emitter or one or more laser emitters) is described or required (within a single claim or across multiple claims) to perform or be configured to perform multiple operations, this language is intended to broadly cover a wide range of architectures and contexts. For example, unless otherwise explicitly required (e.g., by using “first component” and “second component” or other language to distinguish components in the claims), this language is intended to cover a single component that performs or is configured to perform all operations, a group of components that jointly perform or are configured to perform all operations, a first component that performs or is configured to perform a first operation and a second component that performs or is configured to perform a second operation, or any combination of components that perform or are configured to perform operations. For example, when a claim has “one or more components are configured to: perform X; perform Y; perform Z,” the claim should be interpreted as meaning “one or more components configured to perform X; one or more (possibly different) components configured to perform Y; and one or more (possibly different) components configured to perform Z.”

[0072] The elements, actions, or instructions used herein should not be construed as critical or necessary unless explicitly stated otherwise. Similarly, as used herein, the articles “a” and “an” are intended to include one or more items and may be used interchangeably with “one or more.” Furthermore, as used herein, the article “the” is intended to include one or more items associated with the article “the” and may be used interchangeably with “the one or more.” Additionally, as used herein, the term “group” is intended to include one or more items (e.g., related items, unrelated items, or a combination of related and unrelated items) and may be used interchangeably with “one or more.” Where only one item is intended, the phrase “only one” or similar language is used. Furthermore, as used herein, the terms “has,” “have,” “having,” etc., are intended to be open-ended terms. Additionally, the phrase “based on” is intended to mean “at least partially based on” unless explicitly stated otherwise. Likewise, as used herein, the term “or” when used serially is intended to be inclusive and may be used interchangeably with “and / or” unless explicitly stated otherwise (e.g., if used in combination with “any” or “only one”). Furthermore, this document may use spatially relative terms such as “front,” “back,” “below,” “lower,” “above,” “upper,” etc., to facilitate the description of the relationship between one element or feature and another element or feature as shown in the figures. In addition to the orientations shown in the figures, spatially relative terms are intended to cover different orientations of devices, equipment, and / or elements in use or operation. The device may be oriented in other ways (rotated 90 degrees or otherwise), and the spatially relative descriptors used herein may be interpreted accordingly.

Claims

1. A wavelength-tunable semiconductor laser, comprising: A first reflector includes a first integer number of repeating units, wherein each repeating unit in the first integer number of repeating units includes a plurality of phase grating elements, the plurality of phase grating elements having a first plurality of π phase offsets distributed among the plurality of phase grating elements; The second reflector includes a second integer number of repeating units, wherein each repeating unit in the second integer number of repeating units includes a plurality of phase grating elements, the plurality of phase grating elements having a second plurality of π phase offsets distributed among the plurality of phase grating elements; as well as The gain component is positioned between the first and second reflectors. The first plurality of π phase offsets and the second plurality of π phase offsets differ by one phase offset per repeating unit.

2. The wavelength-tunable semiconductor laser according to claim 1, further comprising: The cavity optical length adjustment section, disposed between the gain section and the second reflector or between the gain section and the first reflector, is configured to adjust the optical length of the optical cavity of the wavelength tunable semiconductor laser to adjust the refractive index of the optical cavity and modulate the longitudinal mode spectrum of the light generated by the gain section.

3. The wavelength-tunable semiconductor laser according to claim 1, wherein the first integer number of repeating units is different from the second integer number of repeating units.

4. The wavelength-tunable semiconductor laser according to claim 1, wherein the number of the first plurality of π phase shifts is one less than the number of the second plurality of π phase shifts.

5. The wavelength-tunable semiconductor laser of claim 1, wherein the plurality of phase grating elements in each of the first integer number of repeating units and the second integer number of repeating units include unetched regions paired with etched regions.

6. The wavelength-tunable semiconductor laser of claim 1, wherein the first integer number of repeating units further comprises a plurality of unetched regions.

7. The wavelength-tunable semiconductor laser according to claim 1, wherein: The plurality of phase grating elements in the first integer number of repeating units and the plurality of phase grating elements in the second integer number of repeating units each include a first subset of phase grating elements having a first length and a second subset of phase grating elements having a second length longer than the first length, and The first plurality of π-phase offsets and the second plurality of π-phase offsets are each distributed in a second subset of the phase grating elements having the second length.

8. The wavelength-tunable semiconductor laser according to claim 1, wherein: The first integer number of repeating units in the first reflector are configured to generate a first comb-shaped reflection spectrum having a first plurality of reflection peaks within the target spectral envelope. The second integer number of repeating units in the second reflector are configured to generate a second comb-shaped reflection spectrum having a second plurality of reflection peaks within the target spectral envelope, and The first plurality of reflection peaks have one less reflection peak than the second plurality of reflection peaks.

9. The wavelength-tunable semiconductor laser of claim 8, wherein the first comb reflection spectrum and the second comb reflection spectrum each further include one or more reflection peaks outside the target spectral envelope, the one or more reflection peaks being at least five times smaller than the first plurality of reflection peaks and the second plurality of reflection peaks within the target spectral envelope.

10. The wavelength-tunable semiconductor laser of claim 8, wherein the first plurality of reflection peaks and the second plurality of reflection peaks within the target spectral envelope have a material-based spectral shape that compensates for the gain portion.

11. An optical device, comprising: A first distributed Bragg reflector (DBR) grating includes a first plurality of repeating units, wherein each of the first plurality of repeating units includes a plurality of phase grating elements and a plurality of π phase shifts, wherein: In each of the plurality of repeating units of the first DBR grating, the number of phase grating elements is substantially the same, and In each of the plurality of repeating units of the first DBR grating, the number or position of the plurality of π phase shifts are substantially the same; and The second DBR grating includes a second plurality of repeating units, wherein each of the second plurality of repeating units includes a plurality of phase grating elements and a plurality of π phase shifts, wherein: In each of the plurality of repeating units of the second DBR grating, the number of phase grating elements is substantially the same. In each of the plurality of repeating units of the second DBR grating, the number or position of the plurality of π phase shifts are substantially the same, and The number of the plurality of π-phase offsets in each of the first plurality of repeating units of the first DBR grating is different from the number of the plurality of π-phase offsets in each of the second plurality of repeating units of the second DBR grating.

12. The optical device of claim 11, wherein the DBR gates of the first DBR grating and the second DBR grating having a smaller repeat length have one less phase offset per repeat unit relative to the other DBR gate.

13. The optical device according to claim 11, further comprising: The gain portion between the first DBR grating and the second DBR grating.

14. A method comprising: Light is generated by the gain portion of a wavelength-tunable semiconductor laser; The light is reflected by a first mirror comprising a first integer number of repeating units, wherein each repeating unit comprises a plurality of phase grating elements having a first plurality of π phase offsets distributed among the plurality of phase grating elements. as well as A first portion of the light is output and a second portion of the light is reflected by a second mirror, the second mirror comprising a second integer number of repeating units, each of which comprises a plurality of phase grating elements having a second plurality of π phase offsets distributed among the plurality of phase grating elements. The first plurality of π phase offsets and the second plurality of π phase offsets differ by one phase offset per repeating unit.

15. The method of claim 14, further comprising: The refractive index of the optical cavity of the wavelength-tunable semiconductor laser is adjusted by a cavity optical length adjustment portion arranged between the gain portion and the first mirror or between the gain portion and the second mirror, so as to adjust the refractive index of the optical cavity and modulate the longitudinal mode spectrum of the light generated by the gain portion.

16. The method of claim 14, wherein the first integer number of repeating units is different from the second integer number of repeating units.

17. The method of claim 14, wherein the number of the first plurality of π-phase offsets is one less than the number of the second plurality of π-phase offsets.

18. The method of claim 14, wherein the plurality of phase grating elements in each of the first integer number of repeating units and the second integer number of repeating units include unetched regions paired with etched regions.

19. The method of claim 14, wherein the first integer number of repeating units further comprises a plurality of unetched regions.

20. The method of claim 14, wherein: The plurality of phase grating elements in the first integer number of repeating units and the plurality of phase grating elements in the second integer number of repeating units each include a first subset of phase grating elements having a first length and a second subset of phase grating elements having a second length longer than the first length, and The first plurality of π-phase offsets and the second plurality of π-phase offsets are each distributed in a second subset of the phase grating elements having the second length.

21. The method of claim 14, wherein: The first integer number of repeating units in the first reflector are configured to generate a first comb-shaped reflection spectrum having a first plurality of reflection peaks within the target spectral envelope. The second integer number of repeating units in the second reflector are configured to generate a second comb-shaped reflection spectrum having a second plurality of reflection peaks within the target spectral envelope, and The first plurality of reflection peaks have one less reflection peak than the second plurality of reflection peaks.

22. The method of claim 21, wherein the first comb reflectance spectrum and the second comb reflectance spectrum each further include one or more reflectance peaks outside the target spectral envelope, the one or more reflectance peaks being at least five times smaller than the first plurality of reflectance peaks and the second plurality of reflectance peaks within the target spectral envelope.

23. The method of claim 21, wherein the first plurality of reflection peaks and the second plurality of reflection peaks within the target spectral envelope have a material-based spectral shape that compensates for the gain portion.

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