Semiconductor laser with single-mode emission that can be synchronized over a wide range

Abstract

The present invention provides a widely tunable, single-mode emitting semiconductor laser comprising a semiconductor substrate, a first straight ridge waveguide forming a first coupled cavity, and a second straight ridge waveguide forming a second coupled cavity, the first coupled cavity being separated from the second coupled cavity by a gap. The first and second coupled cavities comprise p-type and n-type contacts for allowing laser currents I1 and I2 to be injected into the first and second coupled cavities, respectively. The first and second coupled cavities comprise first and second heating elements, respectively, for heating the first and second coupled cavities when heating currents H1 and H2 are applied to the first and second heating elements, respectively. A heating element is provided for heating the semiconductor substrate of the semiconductor laser to adjust a reference temperature T of the chip (i.e., the semiconductor substrate).

Description

Technical Field

[0001] Reference to Related Patent Applications This patent application claims the benefit of the prior U.S. Provisional Patent Application No. 63 / 004,816, filed on Apr. 3, 2020, by Automotive Coalition For Traffic Safety, Inc. and Tim Koslowski et al., entitled “WIDELY TUNABLE, SINGLE MODE EMISSION SEMICONDUCTOR LASER”, Attorney Docket No. ACTS-5 PROV, which is incorporated herein by reference.

[0002] The present invention relates generally to lasers, and more particularly, to semiconductor lasers with widely tunable single-mode emission.

Background Art

[0003] Semiconductor lasers with single-mode emission provide a light source for many state-of-the-art applications, from spectroscopy to telecommunications and far beyond. These semiconductor lasers with single-mode emission enable electronic control of the optical output and the wavelength λ emitted.

[0004] For many applications, the wavelength of the emitted laser is measured over a period λ whose width is determined by a tuning rate r given by the following formula min ...λ max and desirably has a wide tuning range.

[0005]

Numbers

[0006] Since general semiconductor lasers emit many wavelengths, an additional wavelength selection mechanism is required to achieve single-mode operation (i.e., emission of a single wavelength λ). Although a wide range of wavelength selection mechanisms exist, all-electronic wavelength selection mechanisms are generally limited to tuning ratios of r ≒ 0.01 or less. Wavelength selection mechanisms corresponding to a wider range of wavelengths generally have a very large parameter space (i.e., many tuning parameters), which complicates the characterization process and control operation of these wavelength selection mechanisms. Therefore, these methods are difficult to implement for mass production.

[0007] More specifically, semiconductor lasers without a wavelength selection mechanism emit light in many modes (i.e., at many wavelengths). For many applications, it is highly desirable to obtain a laser that emits only a single optical mode (i.e., only light of a single wavelength). Although many wavelength selection mechanisms exist, in many cases, it is highly desirable to obtain a monolithic (i.e., on-chip) wavelength selection mechanism. Monolithic (i.e., on-chip) solutions are preferred for several reasons, such as size and mechanical stability. For example, several monolithic wavelength selection mechanisms are known, such as distributed feedback (DFB) gratings, distributed Bragg reflector (DBR) structures, and Vernier tuning of coupled cavities, i.e., coupled resonators.

[0008] In most cases, it is highly desirable to construct a laser with a wide tuning range of wavelengths. To achieve this, it is necessary to be able to control the wavelength selection mechanism. For many applications, it is highly desirable to obtain an all-electronic wavelength selection mechanism. These all-electronic wavelength selection mechanisms are generally based on tuning the refractive index of the laser's waveguide material by utilizing changes in (i) the temperature of the waveguide material and (ii) the density of the injected current. The best side-mode suppression ratio is achieved using lasers with distributed feedback (DFB) gratings and lasers with distributed Bragg reflectors (DBRs). Using these wavelength selection mechanisms, it is possible to achieve a tuning ratio of up to 1 percent (i.e., r ≒ 0.01) for typical operating conditions.

[0009] By adjusting the wavelength selection mechanism (i.e., DFB grating or DBR period) to select not only a single wavelength but also two or more preferred wavelengths, a wider tuning range has been demonstrated. Thus, by the absolute tuning of the effective refractive index of the waveguide material (achieved by changes in the temperature of the waveguide material and the density of the injected current), continuous tuning of the output wavelength is made possible in principle, and by relative tuning (e.g., by selection of the phase relationship), it is possible to construct a multi-segment laser so as to enable the selection of a grating or reflector that is effectively turned on. Combining these two effects (continuous tuning and relative tuning) can, in principle, achieve a much larger tuning range, but in practice, as the tuning range increases, the complexity of fabrication and characterization also increases significantly.

[0010] Another approach to wavelength selection uses the Vernier effect. In this approach, generally, two cavities are defined, each having its own Fabry-Perot wavelength comb. Tuning is then achieved by individually manipulating the refractive indices of the two cavities (i.e., by utilizing changes in the temperature of the waveguide material and the density of the injected current). This selects the emission wavelengths contained in both the Fabry-Perot wavelength comb and the gain range of the gain material used in the laser. This generally enables a much wider tuning range than wavelength selection based on the DFB or DBR wavelength selection mechanism.

[0011] Therefore, known monolithic single-mode semiconductor lasers with a high side-mode suppression ratio (i.e., DFB lasers or DBR lasers) have a small tuning ratio of about 1 percent (i.e., r≈0.01). The operation of lasers having a DFB grating or DBR and capable of selecting multiple wavelengths is usually very complex. As a result, the operating characteristics of the laser become very susceptible to variations in the manufacturing process. Finding the desired protocol for the operating parameters becomes a very complex characterization process in a high-dimensional parameter space (i.e., characterizing each part of the laser, especially the temperature of the waveguide material and the characteristics of the specific injected current density, is a very complex procedure). As a result, the characterization process requires the collection of spectra on a fine grid in a high-dimensional parameter space. The number of elements of this grid increases exponentially with respect to the number of individual parameters, making it impossible to fabricate and control DFB or DBR lasers having the number of parts necessary to stably select many wavelengths.

[0012] Moreover, known mechanisms for implementing wavelength selection by the Vernier tuning principle suffer from a similar problem in that they require many control parameters (especially the temperature of the waveguide material and the injection current density) in order to simultaneously control the output wavelength, output intensity, and side-mode suppression ratio. This also results in a very complex characterization process in a very high-dimensional parameter space, making it difficult to mass-produce devices that can control the output wavelength, output intensity, and side-mode suppression ratio all at the same time. SUMMARY OF THE INVENTION

[0013] The present invention provides a semiconductor laser with single-mode emission that is tunable over a wide range. This semiconductor laser with single-mode emission tunable over a wide range enables full electronic control of the optical intensity, and the wavelength can be electronically tuned generally to a tuning rate r≒0.1 or more, that is, more than 10 times the tuning rate of a conventional single-mode semiconductor laser. The laser of the present invention has a very simple characterization process and an established process route, in contrast to known devices with a wide tuning range, and is thus suitable for mass production.

[0014] Generally, the present invention comprises a monolithic semiconductor laser that provides single-mode emission, a wide tuning range for the wavelength, and simultaneous independent control of the optical intensity. The semiconductor laser is characterized by a semiconductor material containing a layer structure suitable for laser emission configured thereon with two or more linearly aligned ridge waveguides so as to provide two linearly aligned coupled cavities. Moreover, the laser is characterized by (i) the individual controllability of three heating currents, with two power resistors configured in close proximity to the two linear ridge waveguides and one power resistor mounted on the base of the chip (i.e., the monolithic semiconductor laser), and (ii) the individual controllability of the laser currents in the two coupled cavities. Moreover, the laser is characterized by utilizing a gain material having a wide range of gain tuning using temperature variations of the waveguide material.

[0015] More specifically, the present invention is based on the Vernier tuning principle, but the laser is configured to enable simultaneous control of the wavelength and the optical output with a high side-mode suppression ratio for each laser with little need for measurement of the tuning parameters in the characterization process.

[0016] Preferred embodiments of the present invention include a laser having two linearly arranged coupled cavities containing contacts for individually controlling two laser currents (i.e., the injection current density for each coupled cavity). In addition, the laser includes three heaters, the first two heaters being resistors configured on a surface in close proximity to the two coupled cavities such that each effectively thermally contacts only one of the two coupled cavities. The third heater is mounted at the bottom of the laser chip and enables control of the temperature of most of the laser chip on a millisecond time scale.

[0017] The central perception regarding this design is that the wavelength of the mode is

[0018]

Number

[0019] represented by the simple relationship: where a, b1, b2, c1, c2 are the reference temperature T of the chip, the laser currents I1, I2 which are the current densities injected through the two coupled cavities, and the heating currents H1, H2 applied to the two coupled cavities through the adjacent heating elements, and are the tuning parameters of the coupled cavities that describe laser tuning. The fundamental wavelength λ i depends on the relative refractive index of the two coupled cavities and basically depends only on the effective temperatures of the two coupled cavities. All base modes λ iRegarding this, it should be noted that the tuning parameters a, b1, b2, c1, and c2 are the same. The tuning parameters (a, b1, b2, c1, c2) can be measured by measuring the spectrum around the reference point where both the main mode and the side mode are tuned using the tuning parameters (a, b1, b2, c1, c2). The fundamental wavelength is measured by measuring a continuous series with a larger step size after the tuning parameters (a, b1, b2, c1, c2) are known. The key point is to measure only a few tuning parameter lines with high precision to find the tuning parameters (a, b1, b2, c1, c2), and to identify a rather coarse grid to find the fundamental wavelength λ i It is only necessary to find. This significantly reduces the number of operating points that need to be characterized to characterize the laser. In this way, by measuring only five tuning parameters (i.e., a, b1, b2, c1, c2) and a discrete set of the fundamental wavelength λ i the laser can be characterized. The number of fundamental wavelengths depends on the geometry of the laser and the specific application of the laser, and can be as few as four, but can also reach about 20 fundamental modes for a specific purpose. This enables effective characterization of the laser, and thus facilitates mass production of the device.

[0020] In relation to Equation 2 above, λ is the target wavelength to which the laser should be tuned, generally measured in units of nm, λ i is the inherent fundamental wavelength previously determined for the laser (sometimes referred to as the fundamental mode in this specification), generated by some specific combinations of the settings of T, I1, I2, H1, and H2, and generally measured in units of nm, a is a coefficient generally measured in units of nm / absolute temperature (K), T is the change from the temperature setting used to generate the inherent fundamental wavelength λ i at the temperature of the chip, generally measured in absolute temperature (K), b1, b2, c1, and c2 are coefficients generally measured in units of nm / mA, I1, I2, H1, and H2 are changes in the injection current and heating current used for i use, where I1 and I2 are the injection currents for the first and second coupling cavities respectively, and H1 and H2 are the heater currents for the first and second coupling cavities respectively, both measured in mA. It should be understood.

[0021] Therefore, for a given fundamental mode λ with a specific combination of settings of T, I1, I2, H1, and H2 i is known, where the coefficients a, b1, b2, c1, and c2 are determined for the laser, and by appropriately adjusting one or more of the values of T, I1, I2, H1, and / or H2 so that the output of the laser moves from the fundamental wavelength λ i to the target wavelength λ, tuning of the laser from the known fundamental mode λ i to the target wavelength λ can be achieved. Of course, when one or more of T, I1, I2, H1, and / or H2 are adjusted, the refractive index of one or both of the coupling cavities can change, and then it will be understood that in order to achieve the target wavelength λ, it is necessary to further change the values of T, I1, I2, H1, and / or H2.

[0022] Many combinations of T, I1, I2, H1, and H2 can be used to generate a given fundamental wavelength λ i and it will also be understood that many different fundamental wavelengths λ i can be identified for a laser of a particular structure. In practice, it is possible to meaningfully characterize the laser by determining about five, and in some cases more, fundamental wavelengths λ i (possibly up to 20 fundamental wavelengths λ i ) for a particular purpose. Each unique fundamental wavelength λ i reflects the fundamental wavelength λ that reflects the factors related to the intended use of the laser iIt will be appreciated that it is generated by using a specific combination of T, I1, I2, H1 and H2 selected to generate

[0023] Thus, in use, it is desirable to tune the laser to several wavelengths λ, where the laser has a plurality of fundamental wavelengths λ i If it has already been characterized to identify, the user selects a fundamental wavelength λ close to the desired wavelength λ i and sets the values of T, I1, I2, H1 and H2 to the values that generate the selected fundamental wavelength λ i to set the laser to the fundamental wavelength λ i After that, the user adjusts T, I1, I2, H1 and / or H2 as necessary to move the output of the laser from the selected fundamental wavelength λ i to the desired wavelength λ.

[0024] The present invention provides a semiconductor laser with widely tunable single-mode emission, manufactured as a monolithic device, and also features full electronic simultaneous control of wavelength and optical output with a high side-mode suppression ratio, having two advantages over the current state of the art. The monolithic device is preferred because it is small, stable and very robust when implemented in a suitable package. Full electronic control is an easily accessible electronic control that enables highly responsive (i.e., very fast), reliable and relatively low-cost control. Specifically, wavelength tuning based on the cavity heater and the laser current can be achieved on a sub-millisecond time scale, while wavelength tuning based on the substrate heater can be achieved on a millisecond time scale.

[0025] Another advantage of the present invention over the current state of the art is that the laser enables a fairly simple characterization process based on measuring only five tuning parameters (i.e., a, b1, b2, c1, c2 in Equation 2 above) and a small number of fundamental wavelengths (i.e., λ in Equation 2 i ) while achieving a wide tuning range.

[0026] Another advantage of the present invention is that, due to the use of a monolithic fabrication process, the laser can be incorporated into an integrated optical circuit that can easily contain on-chip wavelength and side-mode analyzers. Thus, a simple characterization process enables an on-line calibration mode.

[0027] In one preferred form of the present invention, a semiconductor laser with widely tunable single-mode emission is provided, and this semiconductor laser with widely tunable single-mode emission comprises a semiconductor substrate, a substrate heater for heating the semiconductor substrate, first and second linearly aligned ridge waveguides formed on the semiconductor substrate and separated by a gap so as to form first and second coupled cavities, first and second ridge waveguide heaters for heating the first and second linearly aligned ridge waveguides respectively, first and second p-type contacts respectively formed on the first and second linearly aligned ridge waveguides for applying first and second laser currents to the first and second linearly aligned ridge waveguides, and first and second n-type contacts in electrical communication with the first and second linearly aligned ridge waveguides respectively, and is provided with.

[0028] In another preferred form of the present invention, a method for generating light of a selected wavelength is provided, and this method comprises the step of providing a semiconductor laser, the semiconductor laser comprises a semiconductor substrate, a substrate heater for heating the semiconductor substrate, first and second linearly aligned ridge waveguides formed on the semiconductor substrate and separated by a gap so as to form first and second coupled cavities, first and second ridge waveguide heaters for heating the first and second linearly aligned ridge waveguides respectively, First and second p-type contacts respectively formed on first and second linearly aligned ridge waveguides for applying first and second laser currents to the first and second linearly aligned ridge waveguides, and first and second n-type contacts in electrical communication with the first and second linearly aligned ridge waveguides respectively, preparing a semiconductor laser comprising; characterizing the semiconductor laser by the following equation,

[0029]

Equation

[0030] where a, b1, b2, c1, c2 are the reference temperature T of the semiconductor substrate, the laser currents I1, I2 which are the current densities injected through the first and second coupling cavities respectively, and the heating currents H1, H2 applied to the first and second coupling cavities through the first and second ridge waveguide heaters respectively, and characterizing the semiconductor laser, which are the tuning parameters of the first and second coupling cavities describing laser synchronization using them; adjusting at least one of T, I1, I2, H1 and H2 as necessary to generate light of a selected wavelength; including.

[0031] These and other objects and features of the present invention will be more fully disclosed, i.e., made apparent, by the following detailed description of the preferred embodiments of the present invention to be considered in conjunction with the accompanying drawings, wherein like numerals in the drawings refer to like parts.

Brief Description of the Drawings

[0032]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Embodiments for Carrying Out the Invention

[0033] Referring now to FIGS. 1-10, in one of the preferred forms of the present invention, a semiconductor laser 5 with widely tunable single-mode emission is provided, and the semiconductor laser 5 comprises a semiconductor substrate 10 having an epitaxy enabling semiconductor laser operation, such as a laser diode or a cascade laser. By way of example and not limitation, the semiconductor substrate 10 may comprise a group III / V semiconductor material such as gallium nitride (GaN), gallium arsenide (GaAs), indium phosphide (InP), or gallium antimonide (GaSb), depending on the target wavelength range of the laser. The epitaxy generally comprises a layered structure containing an active region having one or more quantum films, upper and lower claddings, and upper and lower waveguide layers. More specifically, the semiconductor laser 5 with widely tunable single-mode emission has a cubic shape having a lowermost portion formed by the semiconductor substrate 10 and an uppermost portion formed by the upper waveguide layer, and the upper waveguide layer is configured to provide two linear ridge waveguides 15, 20 aligned linearly and perpendicular to four facets 25, 30, 35, 40, where facets 25, 30 form the front and rear facets for the linear ridge waveguide 15, and facets 35, 40 form the front and rear facets for the linear ridge waveguide 20.

[0034] The two linear ridge waveguides 15, 20 preferably have a width and height equivalent to the target wavelength and are preferably spaced apart from each other by a distance D of approximately one-half of the target wavelength. The two linear ridge waveguides 15, 20 are configured to guide the laser mode to the four facets 25, 30, 35, 40. The linear ridge waveguide 15 defines a coupling cavity 45 having facets 25, 30, and the linear ridge waveguide 20 defines a coupling cavity 50 having facets 35, 40. A gap 55 separates the two coupling cavities 45 and 50.

[0035] The two linear ridge waveguides 15, 20 are preferably produced by a material removal process (such as chemical or physical etching) performed from the top. Thus, the remaining material (i.e., the material remaining after etching) defines the two linear ridge waveguides 15, 20. Generally, the lengths of the coupling cavities 45, 50, as well as the gap 55 between the two linear ridge waveguides 15, 20, are defined by a second material removal process, generally etching, such that they are defined with an accuracy of about 10 nanometers. The general lengths of the two coupling cavities 45, 50 are from about 80 nm to about 800 nm. The distributed Bragg reflectors (DBRs) on the facets 25, 30, 35, 40 can also be configured to control such that the reflectivity of each facet and the gap 55 exceeds the value obtained in a single etching step.

[0036] Two heating elements 60, 65 are configured in close proximity to the sides of the two linear ridge waveguides 15, 20, i.e., the heating element 60 extends along the linear ridge waveguide 15 and the heating element 65 extends along the linear ridge waveguide 20. This distance is generally from 1 to several micrometers (Note: This is the respective lateral distance between the linear ridge waveguides 15, 20 and the heating elements 60, 65, and ideally it should be as close as possible, but the minimum distance is limited by the need to insulate the heating elements 60, 65 from laser contact (i.e., p-type contacts 70, 75, see below), and thus a distance of several micrometers, such as 2 micrometers, is required). The two heating elements 60, 65 are generally processed from a highly conductive material (such as titanium (Ti), platinum (Pt), or gold (Au)), and the dimensions are configured such that the total resistance is about several ohms. It is particularly preferred that the heating elements 60, 65 have a "meandering" structure, which promotes a constant resistivity for the respective thermal contact between the two linear ridge waveguides 15, 20 and the two heating elements 60, 65 (see, for example, FIG. 11 which schematically shows the aforementioned "meandering" structure).

[0037] The laser 5 includes two p-type contacts 70, 75 for receiving the top laser currents of the two linear ridge waveguides 15, 20, that is, the p-type contact 70 extends along the linear ridge waveguide 15 and the p-type contact 75 extends along the linear ridge waveguide 20. The two p-type contacts 70, 75 are generally processed using a highly conductive material such as gold (Au) and are respectively connected to two laser current connection pads 80, 85 (that is, the connection pad 80 is connected to the p-type contact 70 for the linear ridge waveguide 15 and the connection pad 85 is connected to the p-type contact 75 for the linear ridge waveguide 20). The two laser current connection pads 80, 85 are arranged on the opposite sides of the laser 5 with respect to the two heating elements 60, 65. The two n-type contacts 86, 87 for the two coupling cavities 45, 50 are processed in a manner understood by those skilled in the art considering the present disclosure at the bottom of the chip (that is, at the bottom of the semiconductor substrate 10, see FIG. 7). By way of example and not limitation, in a preferred form of the present invention, the two n-type contacts are a single shared n-type contact and are at the bottom of the chip that is easily soldered onto the heat spreader (see the heat spreader 105 incorporating the lower, also lower heating element 110). The individual currents are injected through the p-type contacts 70, 75 and do not spread before passing through the active layer (that is, the gain medium in the linear ridge waveguides 15, 20) due to their dimensions and relatively low horizontal conductivity.

[0038] The two heating elements 60, 65 preferably have their own connection pads provided on the opposite sides of the laser current connection pads 80, 85 and may share a common ground pad. By way of example and not limitation, three heating element connection pads 90, 95, 100 may be provided, where the connection pad 95 is the common ground pad, and the connection pads 90, 95 are used to supply current to the heating element 60, and the connection pads 100, 95 are used to supply current to the heating element 65.

[0039] The semiconductor substrate 10 of the laser 5 is preferably mounted on a heat spreader plate 105 that contains a heating element 110 for heating most of the semiconductor substrate 10. The connection pads 115, 120 are used to supply current to the heating element 110.

[0040] If necessary, connection pads 125, 130, 135, 140, 145 are provided on the heat spreader 105 to simplify electrical connections to various components of the semiconductor laser 5. For example, the various connection pads 125, 130, 135, 140, 145 can be connected to various connection pads such as connection pad 80 for the p-type contact 70 of the coupling cavity 45, connection pad 85 for the p-type contact 75 of the coupling cavity 50, connection pad 90 for the heating element 60, connection pad 95 (common ground) for the heating elements 60, 65, and connection pad 100 for the heating element 65, thereby simplifying the electrical connections to these components. Note that the various connection pads do not require a specific bonding method and can be adjusted according to a specific application. However, generally, it is preferred that the common ground for the heating elements 60, 65 is bonded for grounding the chip heating element 110. In other words, generally, it is preferred that the connection pad 120 (ground) of the chip heating element 110 is connected to the connection pad 140 for the common ground 95 of the heating elements 60, 65.

[0041] Therefore, in a preferred embodiment of the present invention, there is provided a semiconductor laser 5 including a semiconductor substrate 10, a first linear ridge waveguide 15 forming a first coupling cavity 45, and a second linear ridge waveguide 20 forming a second coupling cavity 50, wherein the first coupling cavity 45 is separated from the second coupling cavity 50 by a gap 55. The coupling cavities 45, 50 are provided with p-type contacts 70, 75 and n-type contacts (not shown, preferably in the form of a common n-type contact) for enabling laser currents I1, I2 to be injected into the coupling cavities 45, 50, respectively. The coupling cavities 45, 50 are also provided with heating elements 60, 65 for heating the coupling cavities when heating currents H1, H2 are applied to the heating elements 60, 65, respectively. A heating element 110 is provided for heating the semiconductor substrate 10 of the laser 5 so as to adjust the reference temperature T of the chip (i.e., the semiconductor substrate 10).

[0042] The above-described structure provides the semiconductor laser 5 with five "control means" that can be used to adjust the output wavelength of the laser 5, namely, the reference temperature T of the chip (controlled by the current passing through the heating element 110), the laser currents I1, I2 injected through the two coupling cavities 45, 50 (i.e., by the p-type and n-type contacts for the two coupling cavities), and the heating currents H1, H2 applied to the two coupling cavities through the adjacent heating elements 60, 65.

[0043] In essence, in a preferred embodiment of the present invention, the semiconductor laser 5 comprises two coupled cavities 45, 50 which are heated by heating elements 60, 65 respectively. One of the coupled cavities 45, 50 has a length that ensures that the distance between its two Fabry - Perot modes is less than the tuning range. This is generally the shorter cavity (i.e., cavity 50 in the structures shown in FIGS. 1 - 10). The longer cavity (i.e., cavity 45 in the structures shown in FIGS. 1 - 10) may be several times longer than the shorter cavity 50. The two heating elements 60, 65 preferably have a resistance of about 1 - 20 Ω. This ensures that the cavity can be heated with a voltage of less than 10 V and a current of less than 500 mA. The semiconductor laser 5 is constructed on a semiconductor substrate 10 having its own heater 110.

[0044] In a preferred form of the present invention, the semiconductor laser 5 is disposed (e.g., by soldering) on a heat spreader 105.

[0045] The preferred operating mode of the laser is one that characterizes the laser by Equation 2. Each shape of the fundamental mode has a separate fundamental wavelength λ i Thereby, a combination of heater currents H1, H2 can be tuned to sweep the wavelength range. Then, the reference temperature T of the laser is adjusted by changing the current of the heater that heats the semiconductor substrate (i.e., the chip). This shifts the gain maximum value. Then, the laser currents I1, I2 are adjusted to achieve the desired optical output level, which is necessary because the laser efficiency depends critically on the reference temperature T of the chip (i.e., semiconductor substrate 10). Then, the ratio of the two laser currents I1 and I2 is adjusted to achieve the optimal side - mode suppression ratio.

[0046] Next, for example, by changing the combination of the heater currents H1, H2, another sweep can be performed. This process can be repeated until the overall gain of the laser material is covered. The advantage of performing another sweep after fixing the reference temperature T is that the slowest part in the tuning is to achieve a change in the reference temperature T of the chip (i.e., the semiconductor substrate 10), so this tuning process is very fast. Additional information regarding the characterization of semiconductor lasers 1. Background There are numerous possibilities for the structure of a semiconductor laser that can be tuned over a wide range to achieve stable single-mode operation over a wide wavelength range. The requirements for these lasers to be suitable for continuous production assemblies have several constraints, especially as follows.

[0047] 1. Established process: An established process route is required that enables high reproducibility and minimal process variation.

[0048] 2. Monolithic device: It is very advantageous to fabricate the laser as a monolithic device consisting of a single semiconductor chip. This avoids the highly costly and vulnerable alignment steps during production.

[0049] 3. All-electronic control: For field applications, it is highly desirable that tuning be achieved by pure electronic control, which enables the application of relatively inexpensive control electronics.

[0050] 4. Easy characterization: A laser that can be tuned over a wide range has a high-dimensional parameter space (reference temperature and various laser and additional control currents). Direct characterization of a high-dimensional parameter space is difficult to perform. For example, a 5-dimensional parameter space scanned at a resolution of 1% in each parameter requires the characterization of the laser at 10 billion operating points. Therefore, for continuous production, an effective model of the laser that is described by only a few parameters that are easy to measure and have sufficient accuracy and is freely available is essential.

[0051] 2. Effective Model When examining the equilibrium state, it becomes clear that for a given laser, only a small set of certain combinations of somewhat visually accessible amounts influence the equilibrium state. These amounts are the center k o and width Δk of the gain of the laser material, as well as the effective refractive index n i of the cavity part. And also, these amounts are basically functions that depend only on the reference temperature T of the laser chip and the laser current I i and heater current H i at their levels, that is k o (T, I1,..., I n ) and n i (T, I1,..., I n ) where Δk is substantially constant. A sufficiently excellent model of the gain maximum value is expressed by the following formula,

[0052]

Equation

[0053] Assuming that the gain width Δk remains constant is substantially sufficient. The effective refractive index of the laser part can be described by the following formula,

[0054]

Equation

[0055] The effective refractive index of the gap is described by the following formula.

[0056]

Equation

[0057] Therefore, the effective model of the laser is the optical mode that minimizes the mirror loss of the laser, and the vacuum wave number of the optical mode

[0058]

Number

[0059] in the optical mode labeled by, the maximum gain k o from

[0060]

Number

[0061] consists of examining the optical modes within the range of. The modes that minimize the mirror loss are the so-called Fabry-Perot modes. These are standing waves in which the coefficient of the electric field has a minimum value at both laser facets. These modes have a gain length l in which a single photon of that mode is reflected between the cavities gIt is exponentially amplified by stimulated emission in a gain material having an effective index given by the average value thereof. Refer to FIG. 12. In calculating the laser spectrum, it is important to take into account that a combination of a gap and one of the laser portions can be regarded as an optical element that promotes coherent tunneling through one of the laser facets. This coherent tunneling has a periodic dependence on the vacuum frequency of the mode and is thus wavelength-dependent. FIG. 13 schematically shows the transmission of the right facet with respect to the right-moving mode. A similar image also exists for the left facet with respect to the left-moving mode. It is a special advantage of this design that these two reflectivities are the same for the left-moving and right-moving components of the Fabry-Perot mode due to the periodicity of the Fourier transform. This reduces the amount of characterization required to understand the laser's Vernier points. The product of the reflectivities of the two facets makes it possible to calculate the effective number of gain lengths in which photons remain in the laser and is the amplification index of the mode. Only the mode with the maximum amplification index appears within the range of the laser spectrum. This links the model parameters at the operating point to the laser spectrum. It is important to note that an analytical expression for the wavelength dependence of the reflectivities of the two facets is obtained, and that the reflectivity of each facet is well approximated by a periodic function of the wave number.

[0062] 3. Characterization By identifying the mode with the highest reflectivity as the mode that appears in the laser spectrum, it becomes possible to effectively characterize the laser by measuring a small number of spectra. It is rather easy to identify the overall cavity tuning by examining the laser's side-mode spectrum. The overall Fabry-Perot mode of the laser appears as the side mode of the coupled-cavity laser. The vacuum frequency of the Fabry-Perot mode is expressed by the following formula.

[0063]

Equation

[0064] These modes can be regarded as the main mode and the side mode (the mode having a signal exceeding -50 dB) of FIG. 13. Using the formula

[0065]

Equation

[0066] it can be understood that the distance between the side modes is given by dividing the length of the laser by the group index and then doubling it. By measuring a series of changes in one of the laser parameters at a time, the tuning parameter can be easily measured. FIG. 14 shows the tuning of the main mode (red / black) and the side mode (gray scale) when the laser current of the short laser section (i.e., the linear ridge waveguide 20) is changed from 23 mA to 45 mA and the other control parameters are kept constant. The slope of this curve determines the tuning coefficient b2 of the current I2 in Equation 2 (in the example of FIG. 14, the tuning coefficient b2 has a slope of about 0.11 nm / mA for the main mode and the side mode). This slope represents the tuning parameter of the overall optical path length due to current tuning. Similar tuning can be measured when changing the laser current passing through the long laser section (i.e., the linear ridge waveguide 15), and thus the tuning parameter b1 (of the current I1 in Equation 2) of the long laser section is measured. Another important element of the information that can be directly read from FIGS. 13 and 14 is that the intensity of the side mode within the range of the gain region may show periodic modulation. This period is about 17 nm in FIG. 14, and it can be observed that the tuning of the side mode is slower than the tuning of the main mode. This is the resonance transmission effect of the optical path length of the short cavity (i.e., the linear ridge waveguide 20). It can be seen that the main mode is the mode at the maximum value of the periodic reflectivity function of the short cavity (i.e., the linear ridge waveguide 20) where the maximum value intersects the main mode at about 41 mA. This is the Vernier point, i.e., the point where the combined reflectivity of both mirrors reaches the maximum value. By using the periodicity of the reflectivity and the tuning parameter, a number of Vernier points can be predicted.

[0067] The measurement of the tuning of the heater currents H1 and H2 for the linear ridge waveguides 15 and 20 is similar to the measurement of the laser currents I1 and I2 respectively. Figure 15 shows the tuning of the laser with varying heater current for the short linear ridge waveguide 20, depicting only the main side modes. Since the heat generated by the heater increases and decreases quadratically with respect to the current applied to the heater, and the tuning increases and decreases linearly with respect to the temperature change caused by the accumulated heat, it is clearly understood that the tuning increases and decreases quadratically with respect to the heater current as expected. Thus, the tuning parameters c1 and c2 for the heater currents H1 and H2 in Equation 2 can be determined.

[0068] The final element of information necessary to characterize the laser is to determine the gain tuning of the laser. This can be achieved by taking a series of temperatures. Figure 16 shows the tuning of the laser by utilizing the change in the reference temperature of the laser (i.e., changing the current applied to the heating element 110), depicting only the main side modes. It is clearly understood that the optical tuning of the effective cavity length (i.e., the combined length of the linear ridge waveguide 15 and the linear ridge waveguide 20) increases and decreases linearly with respect to temperature from the lines formed by the side modes. Also, by taking into account the fact that the center of the main mode represented in orange or red is on a steeply sloped line, the tuning of the center of the gain of the laser can be read. The optical tuning is

[0069]

Number

[0070] around and the gain tuning is

[0071]

Number

[0072] It is understood that it is around this. In this way, the tuning parameter a for the variable T in Equation 2 can be determined.

[0073] 4. Characterization of a novel laser design versus characterization of lasers mass-produced with the same design Generally, it is desired to understand that there is a difference between the characterization of a novel laser design and the characterization of lasers mass-produced with the same design. In a novel laser design, detailed characterization is generally required because process variations and material properties must be understood to transform effective model parameters. However, once the process variations and process routes of a particular design are understood and lasers of the same design are mass-produced, the characterization problem becomes much simpler. In this case, all that is needed is to measure "fine tuning". Subsequently, simple laser characterization can be reduced to an even shorter program (but a program of the same type as the program described above), which measures only a few characteristics of the laser and finds the tuning parameters and positions of the fundamental wavelength with sufficient accuracy. Alternative structures and operating modes In alternative structures, lasers having three or more coupled cavities, a laser having only one heater in the coupled cavity, and lasers having some coupled cavities with heaters in each cavity or only in a subset of the cavities can be considered. An important part of this design is that the mode equation for the optical mode has scaling symmetry such that a simple equation similar to Equation 2 effectively describes the tuning behavior of the laser.

[0074] In alternative operating modes, the operating mode can be adjusted according to the needs of the application. Specifically, instead of performing sweeps of multiple wavelengths, it may be desirable to scan only a discrete set of wavelengths. Modifications of the preferred embodiment It should be understood that many additional changes in the details, materials, steps and arrangement of parts described and illustrated herein by those skilled in the art to explain the nature of the present invention may be made while still remaining within the principles and scope of the present invention.

Claims

1. A semiconductor laser for single-mode emission, comprising: a semiconductor substrate; a substrate heater for heating the semiconductor substrate; first and second linearly aligned ridge waveguides formed on the semiconductor substrate and separated by a gap so as to form first and second coupling cavities; first and second ridge waveguide heaters for heating the first and second linearly aligned ridge waveguides, respectively; first and second p-type contacts respectively formed on the first and second linearly aligned ridge waveguides for applying first and second laser currents to the first and second linearly aligned ridge waveguides, and first and second n-type contacts in electrical communication with the first and second linearly aligned ridge waveguides, respectively; A semiconductor laser for single-mode emission, comprising the above components.

2. The semiconductor laser for single-mode emission according to claim 1, further comprising a heat spreader, wherein the semiconductor substrate is mounted on the heat spreader, and the substrate heater is further mounted on the heat spreader.

3. Heating the semiconductor substrate, heating the first and second linearly aligned ridge waveguides, and applying first and second laser currents to the first and second linearly aligned ridge waveguides respectively can simultaneously control the output wavelength, output intensity and side mode suppression ratio of the semiconductor laser. The semiconductor laser for single-mode emission according to claim 1.

4. The semiconductor laser for single-mode emission according to claim 1, wherein the semiconductor substrate has a layered structure suitable for laser emission.

5. The semiconductor laser for single-mode emission according to claim 4, wherein the layered structure includes a laser diode.

6. The semiconductor substrate is made of gallium nitride (GaN), gallium arsenide (GaAs), indium phosphide (InP),... ｎ The semiconductor laser for single-mode light emission according to claim 1, comprising a group-III / V semiconductor material selected from the group consisting of indium phosphide (InP) and gallium antimonide (GaSb).

7. The semiconductor laser for single-mode light emission according to claim 1, wherein one of the first and second linearly aligned ridge waveguides has a length exceeding that of the other of the first and second linearly aligned ridge waveguides.

8. The semiconductor laser for single-mode light emission according to claim 1, wherein the first and second ridge waveguide heaters have a meandering structure.

9. A method for generating light of a selected wavelength, comprising: providing a semiconductor laser, the semiconductor laser comprising: a semiconductor substrate; a substrate heater for heating the semiconductor substrate; first and second linearly aligned ridge waveguides formed on the semiconductor substrate and separated by a gap so as to form first and second coupling cavities; first and second ridge waveguide heaters for heating the first and second linearly aligned ridge waveguides, respectively; first and second p-type contacts respectively formed on the first and second linearly aligned ridge waveguides for applying first and second laser currents to the first and second linearly aligned ridge waveguides, and first and second n-type contacts in electrical communication with the first and second linearly aligned ridge waveguides, respectively; providing a semiconductor laser comprising; characterizing the semiconductor laser by the following formula: 【Equation 1】 where λ is the target wavelength, λi is the intrinsic fundamental wavelength determined for the semiconductor laser, and a, b １ 、b ２ 、c １ 、c ２ is the reference temperature T of the semiconductor substrate and the laser currents I １ , I ２ which are the current densities injected through the first and second coupling cavities respectively, and the heating currents H １ , H ２ which are used to describe laser synchronization, and characterizing the semiconductor laser by the tuning parameters of the first and second coupling cavities; adjusting at least one of T, I １ , I ２ , H １ , and H ２ as necessary to generate light of a selected wavelength; A method for generating light of a selected wavelength, comprising. **Claim 10** The semiconductor laser is sweeping a range of wavelengths using a combination of the heater currents H １ , H ２ ; adjusting the reference temperature T of the semiconductor laser by changing the current of the substrate heater to shift the gain maximum; adjusting the laser currents I , I １ , I ２ to achieve a desired optical output level; adjusting the ratio of the two laser currents I １ , I ２ to achieve an optimal side mode suppression ratio; performing another sweep by changing the combination of the heater currents H １ , H ２ ; repeating the above steps until the overall gain of the laser material is covered; A method according to claim 9, characterized by. **Claim 11** **Claim 11** The semiconductor laser is characterized by measuring a discrete set of the synchronization parameter and the fundamental wavelength λ ｉ The method according to claim 9, wherein the discrete set is measured. **Claim 12** The method according to claim 9, wherein the number of the fundamental wavelengths is 4 to 20. **Claim 13** The method according to claim 9, wherein one of the first and second linearly aligned ridge waveguides has a length exceeding that of the other of the first and second linearly aligned ridge waveguides. **Claim 14** The method according to claim 9, wherein the first and second ridge waveguide heaters have a meandering structure.

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