Low-cost discretely tunable laser system with stabilization.

Abstract

The discretely tunable laser system includes a continuously tunable laser emitting a beam tunable among selectable frequencies, the selectable frequencies being separated in frequency by discrete frequency intervals, the discrete frequency intervals including a maximum interval and a minimum interval, the difference between the maximum interval and the minimum interval being 100 MHz or less, and an external stabilization circuit coupled to the continuously tunable laser and a controller. The external stabilization circuit includes a first photodiode generating a first signal corresponding to a portion of the beam, and an interferometer generating a resonance when another portion of the beam is incident on the first photodiode. The resonances are equally spaced in frequency and each define one of the selectable frequencies. A second photodiode generates a second signal corresponding to the transmitted beam generated by the interferometer. The controller tunes the continuously tunable laser among the selectable frequencies based on the first and second signals.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. § 120 to U.S. Provisional Patent Application No. 63 / 313,098, filed February 23, 2022, the contents of which are relied upon and incorporated by reference herein in their entirety. [Technical field]

[0002] The present disclosure relates generally to discretely tunable laser systems with stabilization, and more particularly to a system and method that implements an external optically coupled Fabry-Perot interferometer and proportional-integral-derivative (PID) controller to lock and hold a continuously tunable laser on a Fabry-Perot resonance edge, thereby achieving precise frequency steps. [Background technology]

[0003] Devices such as multi-surface profilers (MSPs), which are frequency-stepping interferometers that provide fast and accurate metrology for components such as semiconductor wafers, have very specific requirements for discrete tuning. Other devices, such as telecommunications devices, use discretely tunable lasers to drive precisely spaced optical channels. In such devices, for example, the laser must be tuned in discrete steps of equally spaced frequency, and the steps should be consistently within + / - 100 MHz of each other. Current MSPs use coupled cavity laser systems. A coupled cavity laser system is a laser diode with an external cavity that feeds back a portion of the laser diode's output to the laser diode by reflecting the light off a holographic grating. The holographic grating is used to tune over the gain bandwidth of the laser diode.

[0004] The internal Fabry-Perot modes of the laser diode define discrete wavelengths and / or frequencies. Although this system works to some extent, it is inherently unstable and chaotic. Small changes in the parameters of the laser diode, for example the reflectivity of the output coupling or the smoothness of the gain profile, can cause unacceptable levels of instability. Changes in the reflectivity of the diffraction grating or the external cavity length can also send the laser into a chaotic state. Additionally, coupled cavity laser systems are very difficult to operate in the stable region since the diode cavity of the diode laser sets the step size, and there are problems with frequency jumps between the diode cavity and the external cavity.

[0005] Another laser system option is a continuously tunable laser. However, to achieve high precision frequency steps, a continuously tunable laser must be coupled with a very expensive optical wavelength frequency measuring instrument called a wavemeter. The advantage of using a continuously tunable laser in MSP is that when properly controlled, it is more stable than a coupled cavity laser system. Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, there is a need for a low-cost system and method for constructing a continuously tunable laser to provide discrete tuning in precise frequency steps over a range of wavelengths. [Means for solving the problem]

[0007] In a first aspect A1, the discretely tunable laser system includes a continuously tunable laser configured to emit a beam tunable among a plurality of selectable frequencies, each of the plurality of selectable frequencies separated in frequency by a plurality of discrete frequency intervals, the plurality of discrete frequency intervals including a maximum frequency interval and a minimum frequency interval, the difference between the maximum frequency interval and the minimum frequency interval being 100 MHz or less. An external stabilization circuit is optically coupled to the continuously tunable laser and electrically coupled to a controller, the controller controlling the tuning of the continuously tunable laser among the plurality of selectable frequencies. The external stabilization circuit includes a first tap configured to split the beam into a first beam and a second beam; a second tap configured to split the second beam into a third beam and a fourth beam; a first photodiode configured to generate a first electrical signal corresponding to a transmitted power of the third beam; a Fabry-Perot interferometer configured to generate a plurality of resonances upon incidence of the fourth beam, the plurality of resonances being equally spaced in frequency, each of the plurality of resonances defining one of a plurality of selectable frequencies; the Fabry-Perot interferometer generating a transmitted beam from the fourth beam; and a second photodiode configured to generate a second electrical signal corresponding to the transmitted power of the transmitted beam from the Fabry-Perot interferometer. The controller is configured to generate one or more tuning signals for tuning the continuously tunable laser to one of a plurality of selectable frequencies based on the first electrical signal and the second electrical signal, and transmit the one or more tuning signals to the continuously tunable laser, thereby causing the continuously tunable laser to emit another beam having one of the plurality of selectable frequencies.

[0008] In a second aspect A2 according to the first aspect A1, the one or more tuning signals adjust one or more tuning elements of a continuously tunable laser, including at least one of a diffraction grating and a laser cavity length, to emit a beam having one of a plurality of selectable frequencies.

[0009] In a third aspect A3 according to the first or second aspect, the controller implements a proportional-integral-derivative (PID) controller configured to control the one or more tuning signals by minimizing an error signal, the error signal being a difference between an amplitude-adjusted first electrical signal corresponding to a transmitted power of a transmitted beam from the Fabry-Perot interferometer and an amplitude-adjusted first electrical signal corresponding to a transmitted power of a third beam.

[0010] In a fourth aspect A4 according to any one of the first to third aspects, the discretely tunable laser system further includes a differential amplifier configured to receive the amplitude-adjusted first electrical signal and the second electrical signal and, in response, generate an error signal.

[0011] In a fifth aspect A5 according to any one of the first to fourth aspects, the external stabilization circuit further includes a third photodiode optically coupled to the transmitted beam from the Fabry-Perot interferometer and configured to generate a third electrical signal corresponding to the transmitted power of the transmitted beam, and the control unit is further configured to implement a second PID control unit configured to receive the third electrical signal and generate a tuning signal for adjusting the cavity length tuning element of the continuously tunable laser based on the RMS value of the third electrical signal and the addition of the error signal.

[0012] In a sixth aspect A6 according to any one of the first to fifth aspects, the discretely tunable laser system further includes an isolator optically coupled to the continuously tunable laser and the first tap to enable light to pass in one direction from the continuously tunable laser to the first tap.

[0013] In a seventh aspect A7 according to any one of the first to sixth aspects, the continuously tunable laser is at least one of a laser tuned using a diffraction grating, a laser tuned using an etalon, and a microelectromechanical systems (MEMs) tunable vertical cavity surface emitting laser (VCSEL).

[0014] In an eighth aspect A8 according to any one of the first to seventh aspects, the Fabry-Perot interferometer is air-spaced.

[0015] In a ninth aspect A9 according to any one of the first to eighth aspects, the plurality of selectable frequencies comprises frequencies in the near infrared band.

[0016] In a tenth aspect A10 according to any one of the first to ninth aspects, the plurality of selectable frequencies includes frequencies between about 272 THz and about 430 THz.

[0017] In an eleventh aspect A11 according to any one of the first to tenth aspects, a power of the first beam is greater than a power of the second beam.

[0018] In a twelfth aspect A12, the discretely tunable laser system includes a continuously tunable laser configured to emit a beam tunable among a plurality of selectable frequencies, each of the plurality of selectable frequencies separated in frequency by a plurality of discrete frequency intervals, the plurality of discrete frequency intervals including a maximum frequency interval and a minimum frequency interval, the difference between the maximum frequency interval and the minimum frequency interval being 100 MHz or less. An external stabilization circuit is optically coupled to the continuously tunable laser and electrically coupled to a controller, the controller controlling tuning of the continuously tunable laser among the plurality of selectable frequencies. The external stabilization circuit includes a first tap configured to split the beam into a first beam and a second beam; a Fabry-Perot interferometer optically coupled to the second beam and configured to generate a plurality of resonances equally spaced in frequency, each of the plurality of resonances defining one of a plurality of selectable frequencies, the Fabry-Perot interferometer generating a transmitted beam and a reflection from the second beam; a first photodiode optically coupled to the Fabry-Perot interferometer through a second tap configured to direct the reflected beam from the Fabry-Perot interferometer to the first photodiode, the first photodiode configured to generate a first electrical signal corresponding to a reflected power of the reflected beam reflected by the Fabry-Perot interferometer; and a second photodiode optically coupled to the transmitted beam from the Fabry-Perot interferometer, the second photodiode configured to generate a second electrical signal corresponding to a transmitted power of the transmitted beam. The controller is configured to generate one or more tuning signals for tuning the continuously tunable laser to one of a plurality of selectable frequencies based on the first electrical signal and the second electrical signal, and transmit the one or more tuning signals to the continuously tunable laser, thereby causing the continuously tunable laser to emit a beam having one of the plurality of selectable frequencies.

[0019] In a thirteenth aspect A13 according to the twelfth aspect A12, the one or more tuning signals adjust one or more tuning elements of a continuously tunable laser, including at least one of a diffraction grating and a laser cavity length, to emit a beam having one of a plurality of selectable frequencies.

[0020] In a fourteenth aspect A14 according to the twelfth or thirteenth aspects, the control unit implements a proportional-integral-derivative (PID) control unit configured to control the one or more tuning signals by minimizing an error signal, the error signal being the difference between a second electrical signal corresponding to the transmitted power of the transmitted beam and a first electrical signal corresponding to the reflected power of the reflected beam reflected by the Fabry-Perot interferometer.

[0021] In a fifteenth aspect A15 according to any one of the twelfth to fourteenth aspects, the discretely tunable laser system further includes a differential amplifier configured to receive the first electrical signal and the second electrical signal and, in response, generate an error signal.

[0022] In sixteenth aspect A16 according to any one of the twelfth to fifteenth aspects, the external stabilization circuit further includes a third photodiode optically coupled to the transmitted beam from the Fabry-Perot interferometer and configured to generate a third electrical signal corresponding to the transmitted power of the transmitted beam, and the control unit is further configured to implement a second PID control unit configured to receive the third electrical signal and generate a tuning signal for adjusting the cavity length tuning element of the continuously tunable laser based on the sum of the RMS value of the third electrical signal and the error signal.

[0023] In a seventeenth aspect A17 according to any one of the twelfth to sixteenth aspects, the discretely tunable laser system further includes an isolator optically coupled to the continuously tunable laser and the first tap to enable light to pass in one direction from the continuously tunable laser to the first tap.

[0024] In an 18th aspect A18 according to any one of the 12th to 17th aspects, the continuously tunable laser is at least one of a laser tuned using a diffraction grating, a laser tuned using an etalon, and a microelectromechanical systems (MEMs) tunable vertical cavity surface emitting laser (VCSEL).

[0025] In a nineteenth aspect A19 according to any one of the twelfth to eighteenth aspects, the Fabry-Perot interferometer is air-spaced.

[0026] In a twentieth aspect A20 according to any one of the twelfth to nineteenth aspects, the plurality of selectable frequencies includes frequencies between about 272 THz and about 430 THz.

[0027] These and further features provided by the embodiments described herein will be more fully understood from the following detailed description taken in conjunction with the drawings.

[0028] The embodiments illustrated in the drawings are exemplary and illustrative and are not intended to limit the subject matter defined by the claims. The following detailed description of exemplary embodiments will be understood when read in conjunction with the accompanying drawings, in which like structure is designated with like reference numerals and in which: [Brief description of the drawings]

[0029] [Figure 1] 1 illustrates a schematic diagram of an example of a continuously tunable laser according to one or more embodiments shown and described herein. [Diagram 2] 1 illustrates a schematic diagram of a discretely tunable laser system according to one or more embodiments shown and described herein. [Diagram 3] 1 illustrates a schematic diagram of another embodiment of a discretely tunable laser system according to one or more embodiments shown and described herein. [Figure 4] 1 illustrates a Fabry-Perot transmitted and reflected power signal plot as a function of frequency in accordance with one or more embodiments shown and described herein. [Diagram 5]1 illustrates a flowchart of an exemplary tuning process in accordance with one or more embodiments shown and described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The present disclosure is provided as an enabling teaching and may be more readily understood by reference to the following description, drawings, examples, and claims. To this end, those skilled in the art will recognize that many variations may be made to the various aspects of the embodiments described herein while still obtaining beneficial results. It will also be apparent that some of the desirable advantages of the present embodiments may be obtained by selecting some of the features without using other features. Thus, those skilled in the art will recognize that many modifications and adaptations are possible and, in some circumstances, even desirable, and are also part of the present disclosure. Thus, unless otherwise specified, it should be understood that the present disclosure is not limited to the particular articles, devices, systems, and methods disclosed. Furthermore, it should be understood that the terms used herein are used for the purpose of describing particular aspects and are not intended to be limiting.

[0031] The embodiments described herein relate to a discretely tunable laser system with an externally coupled stabilization circuit. The system and method implement an externally optically coupled Fabry-Perot interferometer and a proportional-integral-derivative (PID) controller to lock and hold a continuously tunable laser on one of the Fabry-Perot resonance edges, thereby achieving precise frequency steps. The architecture provides a stabilization technique to achieve better laser frequency stability than single-cavity and coupled-cavity laser systems.

[0032] As described in more detail herein, the system and method provide a low-cost stabilization circuit for a continuously tunable laser. This differs from some of the current lasers implemented in MSP systems, for example, which include coupled cavity lasers that are discretely tunable by nature. The problem with single cavity lasers is that they are not discretely tuned over precisely spaced frequencies. To address this problem, a high-resolution wavemeter is needed to precisely measure the output frequency or wavelength during tuning. High-precision wavemeters are expensive and can increase the cost of a system by 50% or more.

[0033] Instead, embodiments of the present disclosure use a Fabry-Perot interferometer externally coupled to a continuously tunable laser to provide a series of equally spaced transmission peaks (referred to herein as "equally spaced transmission peaks"). Signals corresponding to the equally spaced transmission peaks are generated through a light detection sensor, e.g., a photodiode, to achieve precise discrete tuning of the continuously tunable laser. In embodiments in which the Fabry-Perot interferometer is air-spaced, the transmission peaks are precisely equally spaced because the optical dispersion of air is very low. The continuously tunable laser may be tuned by monitoring the optical power transmitted through the Fabry-Perot interferometer and minimizing (e.g., zeroing) the error signal as described herein. In some embodiments, as described in more detail herein, the error signal is defined by the difference between the transmitted and reflected beams generated by the Fabry-Perot interferometer, or the difference between the transmitted beam generated by the Fabry-Perot interferometer and a portion of the beam emitted by the continuously tunable laser. The error signal may be minimized by tuning the continuously tunable laser to lock and hold the continuously tunable laser on one of the Fabry-Perot resonance edges using a PID feedback controller, as described in more detail herein.

[0034] The discretely tunable laser systems and methods of the present disclosure will now be described in more detail with reference to the drawings, in which like numbers refer to like structures.

[0035] Referring to FIG. 1, an example of a continuously tunable laser 100 is shown. The discretely tunable laser system embodiments described herein include low-cost stabilization circuitry for a continuously tunable laser (e.g., a broadly tunable laser) so that precise (uniform) frequency steps can be achieved while tuning over multiple selectable frequencies (also referred to herein as wavelength ranges). MSP devices typically operate at multiple selectable frequencies, e.g., in the near infrared band, and optionally at wavelengths ranging from about 272 THz to about 430 THz, or, in wavelength terms, from 700 nm to 1100 nm. Currently, there are few, if any, International Traffic in Arms Regulations (ITAR) approved cameras for the near infrared outside of the above range, so the wavelength range of 700 nm to 1100 nm may be optimal for MSP devices. In other words, the wavelength of light used to measure flatness and thickness may be appropriately matched to a commercially available imaging system. However, it should be understood that for other devices and applications, different ranges of wavelengths may be used. For example, the telecommunications industry uses a low loss wavelength region of 1260 nm to 1625 nm, and other devices may use wavelength ranges higher or lower than that range. For purposes of this disclosure, MSP devices and wavelengths from 700 nm to 1100 nm will be used. Furthermore, for purposes of explanation and consistency when using the term "frequency," it should be understood to mean either the frequency or the equivalent wavelength.

[0036] As shown, the continuously tunable laser 100 includes a laser diode 110 that generates light having a wavelength that spans hundreds of nanometers. The laser diode 110 includes a cavity mirror 112 and an opposing cavity mirror 114, with the space between them defining a cavity length that may reflect and direct the light generated by the laser diode 110 in a particular direction. In this illustration, a lens 120 may be positioned in the path of light from the laser diode. The lens 120 may operate to focus, collimate, and / or filter the light. A diffraction grating 130 is used to adjust the frequency of the energy (e.g., light beam 150) that ultimately propagates from the continuously tunable laser 100. The diffraction grating 130 may include one or more tuning elements, such as a stepper motor 132 for coarsely adjusting the angle of the diffraction grating, and one or more piezoelectric elements (e.g., piezoelectric actuators) for finely adjusting the angle of the diffraction grating. As described in more detail herein, the controller generates one or more tuning signals to adjust the tuning elements of the continuously tunable laser 100 so that the selected frequency is maintained. The angle of the diffraction grating 130 may be adjusted from time to time to filter out undesired frequencies and pass desired frequencies, and a beam steering mirror 140 may be placed on the continuously tunable laser 100 to direct the beam 150 away from the continuously tunable laser 100. It should be understood that the continuously tunable laser 100 may include different components and / or different configurations depending on the type of continuously tunable laser 100. The continuously tunable laser 100 may be a grating tuned laser similar to that shown in FIG. 1, an etalon tuned laser, a microelectromechanical system (MEMs) tuneable vertical cavity surface emitting laser (VCSEL), or other types of MEMs tuneable VCSEL.

[0037] One challenge for any continuously tunable laser 100 is to maintain stable single-longitudinal mode operation over the tuning range. The reason is that the gain medium usually has high optical dispersion. This means that the cavity length 113 varies with frequency. To eliminate mode hopping between multiple longitudinal modes, the cavity length should be controlled to keep the cavity mode tuned to the desired wavelength and / or frequency. In the case of a laser diode, this may be done by varying the injection power or by varying the position of one of the cavity mirrors 112 and the opposing cavity mirror 114. This is not necessary in the case of an ultrashort cavity MEMs tunable VCSEL. However, if it is necessary to prevent mode hopping, the controller may monitor a photodiode optically coupled to the transmitted portion of the transmitted beam emerging from the Fabry-Perot interferometer, as described in more detail herein.

[0038] 2 and 3, block diagrams of exemplary discretely tunable laser systems 200, 202 are shown. Referring to FIG. 2, the discretely tunable laser system 200 includes a continuously tunable laser 100, an external stabilization circuit, and a controller 210. The external stabilization circuit is optically coupled to the continuously tunable laser 100. The controller 210 is electrically coupled to components of the external stabilization circuit and is further electrically coupled to the continuously tunable laser 100.

[0039] The controller 210 may be any device or combination of components including the processor 212 and the memory element 214. In some embodiments, the controller 210 may be comprised of analog elements. The processor 212 of the discretely tunable laser system 200 may be any device capable of executing a set of machine-readable instructions stored in the memory element 214. Thus, the processor 212 may be a microcontroller, an integrated circuit, a microchip, a field programmable gate array, a computer, or any other computing device. The processor 212 is communicatively coupled to other components of the discretely tunable laser system 200 by one or more conductive paths. Thus, the one or more conductive paths may communicatively couple any number of the processors 212 to each other, allowing the components coupled to the one or more conductive paths to operate in a distributed computing environment. In particular, each component may operate as a node that may transmit / receive data. Although the embodiment shown in FIG. 2 includes a single processor 212, other embodiments may include more than one processor 212.

[0040] The memory element 214 of the discretely tunable laser system 200 is communicatively coupled to the processor 212. The memory element 214 may be a non-transitory computer readable memory that may be implemented, for example, as a RAM, a ROM, a flash memory, a hard drive, or any non-transitory memory device capable of storing machine-readable instructions such that the machine-readable instructions may be accessed and executed by the processor 212. The set of machine-readable instructions may include logic or algorithms written in any programming language of any generation (e.g., 1GL, 2GL, 3GL, 4GL, or 5GL), such as a machine language that may be directly executed by the processor 212, or assembly language, object-oriented programming (OOP), scripting language, microcode, etc., and may be compiled or assembled into machine-readable instructions and stored in the memory element 214. Alternatively, the set of machine-readable instructions may be written in a hardware description language (HDL), such as logic implemented via either a field programmable gate array (FPGA) configuration or an application specific integrated circuit (ASIC), or their equivalents. Thus, the functionality described herein may be implemented in any conventional computer programming language, as pre-programmed hardware elements, or as a combination of hardware and software elements. The discretely tunable laser system 200 shown in FIG. 2 includes a single memory element 214, while other embodiments may include more than one memory element 214.

[0041] In an embodiment, the continuously tunable laser 100 generates an output beam 5. In some embodiments, the continuously tunable laser 100 is optically coupled to an isolator 220. The isolator 220 blocks reflections from either the laser output optics, the Fabry-Perot interferometer 250, or other optical elements in the beam path. The isolator 220 receives the output beam 5 and also propagates the output beam 10 through the isolator 220 to prevent reflections from returning to the continuously tunable laser 100. The output beam 5, 10 generated by the continuously tunable laser 100 further propagates to a first tap 230A disposed in the beam path. The first tap 230A splits the output beam 10 into a first portion for lasing (e.g., lasing beam 15) and a second portion for tuning (referred to herein as second beam 20). The power of the first portion of the output beam 10 (eg, lasing beam 15) is greater than the power of the second portion of the output beam 10 (eg, second beam 20).

[0042] In the embodiment of the discretely tunable laser system 200 shown in FIG. 2, the second tap 230B is disposed in the path of the second beam 20. The second tap 230B splits the second beam 20 into two separate beams, a third beam 30 and a fourth beam 25. The second tap 230B directs the fourth beam 25 to a Fabry-Perot interferometer 250 optically coupled to the continuously tunable laser 100. Notably, it is noted that the Fabry-Perot interferometer 250 is not an internal element of the continuously tunable laser 100. The Fabry-Perot interferometer 250 is disposed external to the continuously tunable laser 100. Additionally, the second tap 230B optically couples and directs the third beam 30 to a first photodiode 240A (PD1). PD1 240A generates a first electrical signal 35 that corresponds to the transmitted power of the third beam 30. As used herein, with respect to an electrical signal generated by a photodiode from a beam, the term "corresponding to the power (or transmitted power) of" means that the magnitude of the electrical signal (e.g., voltage or current) is proportional to or has a 1:1 relationship with the power (or transmitted power) of the beam.

[0043] The Fabry-Perot interferometer 250 uses the phenomenon of multiple beam interference that occurs when light shines through a cavity bounded by two reflective parallel surfaces 250A, 250B. Each time the light hits a surface, part of it is transmitted and the rest is reflected. The net effect is that the single beam is split into multiple beams that interfere with each other. If the additional optical path length of the reflected beams (due to multiple reflections) is an integer multiple of the frequency of the light, then the reflected beams will constructively interfere. The Fabry-Perot interferometer 250 provides equally spaced resonant frequencies to which the system can be tuned.

[0044] As the frequency of the continuously tunable laser 100 is changed, the peaks corresponding to the resonant frequencies are transmitted through the Fabry-Perot interferometer 250 and the non-resonant frequencies are reflected. This is shown in FIG. 4 and described in more detail herein. The transmitted resonant frequencies are referred to herein as the transmitted beam 40. The transmitted beam 40 from the Fabry-Perot interferometer 250 is incident on a second photodiode 240B (PD2). PD2 240B generates a second electrical signal 45 corresponding to the transmitted power of the transmitted beam 40. The second electrical signal 45 is a voltage signal and may also be referred to as a percent transmission.

[0045] In some embodiments, the third tap 230C taps off a portion of the power of the transmitted beam 40 and directs the tapped transmitted beam 60 to a third photodiode 240C (PD3). The PD3 240C generates a third electrical signal 65 that corresponds to the transmitted power of the tapped transmitted beam 60. That is, the PD3 240C provides a reference value for the optical power that is used to measure the noise of the laser output.

[0046] The discretely tunable laser system 200 further includes a differential amplifier 260. In the embodiment shown in FIG. 2, the second tap 230B directs the fourth beam 25 to the optically coupled Fabry-Perot interferometer 250. Since the fourth beam 25 is not a reflected or transmitted signal from the Fabry-Perot interferometer 250, an amplitude adjustment of the first electrical signal 35 corresponding to the fourth beam 25 is necessary before the first electrical signal 35 and the second electrical signal 45 are compared by the differential amplifier 260. Therefore, the multiplication circuit 270 injects an adjustment value A into the first electrical signal 35 to generate an amplitude-adjusted first electrical signal 35A that is fed to the differential amplifier 260. The adjustment value A is set so that the amplitude-adjusted first electrical signal 35A is 1 / 2 the peak value of the second electrical signal 45.

[0047] The differential amplifier 260 (e.g., an operational amplifier) ​​receives the amplitude-adjusted first electrical signal 35A (e.g., A*(PD1)) and the second electrical signal 45 (e.g., PD2) and determines the difference between the second electrical signal 45 (e.g., PD2) and the amplitude-adjusted first electrical signal 35A (e.g., A*PD1). The output of the differential amplifier 260 is an error signal 50 that is sent to the controller 210. In some embodiments, the controller 210 receives the amplitude-adjusted first electrical signal 35A (e.g., A*(PD1)) and the second electrical signal 45 (e.g., PD2). The controller 210 determines the difference between the two signals, e.g., the second electrical signal 45 minus the amplitude-adjusted first electrical signal 35A, to calculate the error signal 50.

[0048] To set the adjustment value A, the user may set the output frequency of the beam 5 of the continuously tunable laser 100 to correspond to the resonant frequency of the Fabry-Perot interferometer 250 and measure the peak values ​​of the first electrical signal 35 and the second electrical signal 45 (e.g., PD2). The adjustment value A may then be set by solving the equation A*PD1(peak voltage)=½PD2(peak voltage) for A. The adjustment value A may be set by the controller 210 through the signal 80. In some embodiments, the amplitude-adjusted first electrical signal 35A provided to the negative terminal of the differential amplifier 260 is set to 50% of the maximum value of the signal 45, thereby making the implementation of the adjustment value A described herein one way of achieving the setting of the value of the signal provided to the differential amplifier.

[0049] One or more proportional-integral-derivative (PID) controllers are implemented to generate tuning signals for adjusting the continuously tunable laser 100. For example, the controller 210 can be configured to implement a PID controller. A first PID controller is implemented to minimize the error signal 50 by generating one or more tuning signals 70, 75 that adjust tuning elements of the continuously tunable laser 100 to minimize the error signal 50. The tuning elements of the continuously tunable laser 100 include at least one of a diffraction grating, a filter, a laser cavity length, or other optical elements known in the art for varying the frequency of the beam 5 by the continuously tunable laser 100.

[0050] In an embodiment, when the control unit 210 generates one or more tuning signals 70, 75, the control unit 210 transmits the one or more tuning signals 70, 75 to the continuously tunable laser 100 to adjust one or more tuning elements, further causing the continuously tunable laser to emit a beam 5 having a selectable frequency.

[0051] The PID controller operates by locking to the zero crossings of the error signal 50. By subtracting the second electrical signal 45, corresponding to the transmitted power of the transmitted beam 40, from the amplitude adjusted first electrical signal 35A, corresponding to the adjusted transmitted power of the third beam 30, an error signal 50 is generated, which can be minimized (e.g., to zero) through tuning of the frequency of the beam 5 of the continuously tunable laser 100. This occurs on both the rising and falling edges of the Fabry-Perot resonance, as described in more detail herein.

[0052] In some embodiments of the discretely tunable laser system 200, the controller 210 may also receive a third electrical signal 65 generated by a third photodiode 240C (PD3), which corresponds to a branched portion of the transmitted beam 40 sent from the Fabry-Perot interferometer 250, referred to herein as the branched transmitted beam 60. The third electrical signal 65 provides a reference value for power and may be used to measure noise in the output of the continuously tunable laser 100. The controller 210 is configured to monitor the noise or root mean square (RMS) value of the third electrical signal 65 for evidence of mode hopping. Using the third electrical signal 65, the controller 210 may improve control of the continuously tunable laser 100 by adding the RMS of the third electrical signal 65 to the error signal 50. A second PID controller may be implemented to null out noise in the third electrical signal 65 through a feedback loop that includes controlling and / or generating a tuning signal 75 for adjustment of a cavity length tuning element of the continuously tunable laser 100. This ensures that the cavity mode is tuned to a desired selectable frequency.

[0053] In some cases, mode pulling can occur when operating the first and second PID controls. Mode pulling occurs in laser systems that do not have an ultrashort cavity and include a wavelength selection element. That is, the laser can lase at a wavelength between the wavelength of the tuning element and the wavelength of the closest cavity mode. Thus, multiple PID controls can cancel each other out, resulting in an unstable output wavelength. To prevent this, one PID control can be filtered to a low frequency and set to have a much slower response than the other PID controls. For example, the response speeds of the controls can differ, for example, by at least a factor of 10. As with the wavelength tuning element, the cavity length tuning element can be calibrated during manufacturing and the results can be used to provide a feedforward signal for faster and more robust tuning.

[0054]

[0033] Referring now to Figure 3, there is shown another exemplary block diagram of a discretely tunable laser system 202. The discretely tunable laser system 202 shown in Figure 3 is configured similarly to the discretely tunable laser system 200 shown in Figure 2. Therefore, only the differences between the two discretely tunable laser systems 200, 202 will be described.

[0055] In particular, while the second tap 230B of the discretely tunable laser system 200 shown in Figure 2 is positioned to split the second beam 20, the second tap 232B of the discretely tunable laser system 202 shown in Figure 3 is positioned to split a reflected optical signal, referred to herein as a reflected beam 26 from the Fabry-Perot interferometer 250. The second tap 232B directs the second beam 20 to the optically coupled Fabry-Perot interferometer 250 as a fourth beam 25, and further directs the reflected beam 26 generated by the Fabry-Perot interferometer 250 to an optically coupled first photodiode 240A (PD1). The PD1 240A generates a first electrical signal corresponding to the power of the reflected beam 26 from the Fabry-Perot interferometer 250.

[0056] 3, the differential amplifier 260 receives the first electrical signal 35 (e.g., generated by PD1) and the second electrical signal 45 (e.g., generated by PD2) and determines the difference between the second electrical signal 45 and the first electrical signal 35. For the discretely tunable laser system 202, there is no need to pre-adjust the first electrical signal 35 with the adjustment value A as described for the first discretely tunable laser system 200.

[0057] Referring to FIG. 4, exemplary signal plots of Fabry-Perot transmitted and reflected power as a function of frequency are shown. The transmission plot (top) shows an example of the second electrical signal 45 generated by the second photodiode 240B (PD2). The reflection plot (bottom) shows an example of the first electrical signal 35 generated by the first photodiode 240A (PD1) implemented in the discretely tunable laser system 202. Additionally, although not shown, a plot of the amplitude-adjusted electrical signal 35A generated by the first photodiode 240A (PD1) implemented in the discretely tunable laser system 200 may generally resemble a bell-shaped curve shape extending over the tuning range selected for the continuously tunable laser 100. That is, the power represented by the bell-shaped curve shape is proportional to the output power of the continuously tunable laser 100 and is independent of the Fabry-Perot transmitted and reflected power. For example, the center of the tuning range of the output beam 5, a portion of which is branched off and directed to the first photodiode 240A (PD1) implemented in the discrete tunable laser system 200, contains most of the power that decreases as one approaches both ends of the tuning range.

[0058] With particular reference to the plot shown in Figure 4 and the discrete tunable laser system 202 shown in Figure 3, the reflection percentage and the transmission percentage correspond to the voltage signal values ​​generated by the first photodiode 240A (PD1) for the reflected signal and the second photodiode 240B (PD2) for the transmitted signal, respectively, where the reflection percentage and the transmission percentage are relative to the maximum voltage signal values ​​from the first photodiode 240A (PD1) and the second photodiode 240B (PD2), respectively. The PID controller operates by locking to the zero crossings of the error signal. In some embodiments, locking can be accomplished by selecting a particular frequency point 45' along a rising edge 45L, a peak 45P, or a falling edge 45T of the transmitted signal (e.g., the second electrical signal 45) and identifying the frequency point 35' that corresponds to the reflected signal (e.g., the first electrical signal 35 for the discrete tunable laser system 202) or the amplitude-adjusted electrical signal 35A generated by the first photodiode 240A (PD1) implemented in the discrete tunable laser system 200. By subtracting the transmitted signal (e.g., the second electrical signal 45) from the reflected signal (e.g., the first electrical signal 35) or vice versa, the controller generates an error signal that is zero when they are equal (e.g., the reflection percentage and the transmission percentage are both equal at 50%). Depending on the frequency of the output beam 5 (e.g., also referred to as the wavelength of the beam 5) and the signal used to drive the tuning element that controls the cavity stability, one can select which edge (e.g., rising edge 45L, 35L or falling edge 45T, 35T) to lock.

[0059] Adjusting one or more tuning elements of the continuously tunable laser 100 changes the frequency of the beam 5 generated by the continuously tunable laser 100. The frequency of the beam 5 of the continuously tunable laser 100 that minimizes the error signal 50 is referred to herein as the "selectable frequency." As can be seen from FIG. 4, the continuously tunable laser 100 has multiple selectable frequencies. Each resonance peak 45P of the Fabry-Perot interferometer 250 has two selectable frequencies, one on the rising edge 45L and the other on the falling edge 45T. To maintain uniform frequency spacing when discretely tuning the continuously tunable laser 100, it is preferable that all the selectable frequencies are associated with only the rising edge 45L or only the falling edge 45T of each resonance peak 45P. In the exemplary embodiment of FIG. 4, the selectable frequencies are defined by the rising edge 45L of the resonance peak 45P as shown. 4 shows four resonant peaks 45P with selectable frequencies indicated by dots (e.g., frequency points 45') on the rising edge 45L of each of the four resonant peaks 45P. The labels "df1", "df2" and "df3" define frequency intervals between consecutive resonant peaks 45P in the set of four shown resonant peaks 45P. The frequency intervals df1, df2 and df3 are equal within a tolerance described herein (e.g., within + / - 100 MHz or less).

[0060] The device of the laser that controls the wavelength of the beam 5 is typically a filter, such as a Fabry-Perot etalon or a diffraction grating. MEMs-tunable VCSELs can also be used. One mirror of the laser cavity is driven by a MEMs actuator. This changes the cavity length and shifts the wavelength at which the VCSEL can lase. Because the cavity is very short, the free spectral spacing of these Fabry-Perot cavity modes is larger than the gain bandwidth of the laser. That means that the only modes that will lase are those within the gain bandwidth of the VCSEL diode, and the wavelength of this mode can be tuned by simply tuning the cavity length with a MEMs actuator.

[0061] Whatever mechanism is used to tune the laser wavelength of the continuously tunable laser, the tuning signal is driven by the output of a PID controller (e.g., implemented by the controller 210), and the error signal is the difference between the electrical signals associated with the reflected and transmitted power from an externally located Fabry-Perot interferometer 250. By doing so, the discretely tunable laser system 200, 202 can precisely jump and lock to a wavelength. The wavelength or frequency interval is specified by the resonance interval of the external Fabry-Perot interferometer 250. The Fabry-Perot interferometer 250 can have a frequency interval in the range of several gigahertz, tens of gigahertz, or hundreds of gigahertz. For example, the frequency interval can be 200 MHz, 250 MHz, 500 MHz, 1 GHz, 2 GHz, 5 GHz, 10 GHz, 20 GHz, 25 GHz, 30 GHz, 35 GHz, 40 GHz, 50 GHz, or a value between 200 MHz and 50 GHz. As described herein, the present application aims to lock the frequency of the output beam 5 of the continuously tunable laser 100 to a series of frequencies separated by regular frequency intervals within + / - 100 MHz, which may be, for example, within + / - 100 MHz, or within + / - 75 MHz, or within + / - 50 MHz, or within + / - 25 MHz, or within + / - 10 MHz. That is, for example, a higher frequency output beam 5 will provide a larger range, while a lower frequency output beam 5 may provide a narrower range, such as + / - 10 MHz, which may be desirable and provided in this embodiment. In other words, as the frequency of the output beam 5 decreases, the range is correspondingly adjusted.

[0062] When the Fabry-Perot interferometer 250 is based on an air-spaced etalon, there is virtually zero dispersion over the frequency tuning range, facilitating consistent frequency spacing over the tuning range of the continuously tunable laser 100. This is important for measurement devices that rely on interferometric measurements at equally spaced frequencies. If the Fabry-Perot interferometer is highly fine, the resonances will be very sharp, allowing it to be locked to spaced frequencies with great consistency and precision over the tuning range.

[0063] Referring to FIG. 5, a flow chart 500 of an exemplary tuning process is shown. In block 510, the continuously tunable laser 100 is started. The start process includes, for example, providing power to the laser and providing a tuning signal that adjusts a tuning element to a start position for an output beam of an output at a selected frequency. In block 520, the PID control of the controller 210 may be activated. As described herein, the PID control generates a tuning signal that adjusts the tuning element of the continuously tunable laser 100 to bring the error signal to the zero crossing frequency. In block 530, the PID control may be deactivated. Once deactivated, the controller 210 may coarsely adjust the tuning element of the continuously tunable laser 100 to the next frequency in block 540. In block 550, the PID control of the controller 210 may be reactivated to re-fine tune the continuously tunable laser 100. At block 560, the PID controller is again deactivated and if further tuning is required, the tuning process returns to block 540. This process is repeated over the tuning range / bandwidth of the continuously tunable laser 100. The approximate correct settings for the drive signals of the wavelength / frequency tuning elements may be previously determined through a calibration process and that signal may be used as a feedforward value and added to the output of the PID controller.

[0064] It should be understood that the continuously tunable laser is optically coupled to an externally located Fabry-Perot interferometer to provide a discretely tunable laser system with stabilization. For example, in some embodiments, the discretely tunable laser system includes a continuously tunable laser configured to output a beam tunable among a plurality of selectable frequencies, the selectable frequencies being frequencies separated by a plurality of discrete frequency intervals, the plurality of discrete frequency intervals including a maximum frequency interval and a minimum frequency interval, the difference between the maximum frequency interval and the minimum frequency interval being 100 MHz or less. An external stabilization circuit is optically coupled to the continuously tunable laser and electrically coupled to a controller, the controller controlling tuning of the continuously tunable laser among the plurality of selectable frequencies. The external stabilization circuit includes a first tap configured to split the beam into a first beam and a second beam, a second tap configured to split the second beam into a third beam and a fourth beam, a first photodiode configured to generate a first electrical signal corresponding to a transmitted power of the third beam, a Fabry-Perot interferometer configured to generate a plurality of equally spaced resonances in frequency upon incidence of the fourth beam, each of the plurality of resonances defining one of a plurality of selectable frequencies, generating a transmitted beam from the fourth beam, and a second photodiode configured to generate a second electrical signal corresponding to a transmitted power of the transmitted beam from the Fabry-Perot interferometer. The controller is configured to generate one or more tuning signals based on the first electrical signal and the second electrical signal to tune the continuously tunable laser to one of the plurality of selectable frequencies and to transmit the one or more tuning signals to the continuously tunable laser, thereby causing the continuously tunable laser to output a beam having one of its plurality of selectable frequencies.

[0065] In some embodiments, the controller implements a proportional-integral-derivative (PID) controller configured to generate one or more tuning signals by minimizing an error signal, the error signal being the difference between a second electrical signal corresponding to the transmitted power of the transmitted beam and a regulated multiple of the first electrical signal corresponding to the transmitted power of the second portion of the beam. Furthermore, in some embodiments, the external stabilization circuit further includes a third photodiode optically coupled to the transmitted beam from the Fabry-Perot interferometer. The third photodiode may be implemented to improve the locking of the PID controller; that is, for good locking, the same percentage of output power needs to be maintained. Through the electrical signal generated by the third photodiode, the controller may monitor the output power, and if the power rises or falls between steps, then the controller may linearly adjust the locking point by increasing or decreasing the same percentage.

[0066] The terms "substantially" and "about" are used herein to describe the inherent degree of uncertainty that may result from any quantitative comparison, value, measurement, or other representation. These terms are also used herein to describe the degree to which a quantitative representation may vary from the stated basis without changing the basic function of the subject matter.

[0067] While particular embodiments have been illustrated and described herein, it should be understood that various other modifications and changes can be made without departing from the spirit and scope of the claimed subject matter. Moreover, although various aspects of the claimed subject matter have been described herein, such aspects need not be utilized in combination. It is therefore intended that the appended claims cover all such modifications and changes that are within the scope of the claimed subject matter.

[0068] Preferred embodiments of the present invention will be described below in detail.

[0069] EMBODIMENT 1 In a discretely tunable laser system, a continuously tunable laser configured to emit a beam tunable among a plurality of selectable frequencies, each of the plurality of selectable frequencies being separated in frequency by a plurality of discrete frequency intervals, the plurality of discrete frequency intervals including a maximum frequency interval and a minimum frequency interval, the difference between the maximum frequency interval and the minimum frequency interval being 100 MHz or less; an external stabilization circuit optically coupled to the continuously tunable laser and electrically coupled to a controller, the controller controlling tuning of the continuously tunable laser among the plurality of selectable frequencies; the external stabilization circuit comprising: a first tap configured to split the beam into a first beam and a second beam; a second tap configured to split the second beam into a third beam and a fourth beam; a first photodiode configured to generate a first electrical signal corresponding to a transmitted power of the third beam; a Fabry-Perot interferometer configured to generate a plurality of resonances upon incidence of the fourth beam, the plurality of resonances being equally spaced in frequency, each of the plurality of resonances defining one of the plurality of selectable frequencies, and generating a transmitted beam from the fourth beam; a second photodiode configured to generate a second electrical signal corresponding to a transmitted power of the transmitted beam from the Fabry-Perot interferometer; and The control unit is generating one or more tuning signals for tuning the continuously tunable laser to one of the plurality of selectable frequencies based on the first electrical signal and the second electrical signal; a discretely tunable laser system configured to transmit the one or more tuning signals to the continuously tunable laser, thereby causing the continuously tunable laser to emit another beam having the one of the plurality of selectable frequencies.

[0070] EMBODIMENT 2 2. The discretely tunable laser system of embodiment 1, wherein the one or more tuning signals adjust one or more tuning elements of the continuously tunable laser, including at least one of a diffraction grating and a laser cavity length, to emit the beam having the one of the plurality of selectable frequencies.

[0071] EMBODIMENT 3 3. The discrete tunable laser system of embodiment 1 or 2, wherein the controller implements a proportional-integral-derivative (PID) controller configured to control the one or more tuning signals by minimizing an error signal, the error signal being a difference between an amplitude-adjusted first electrical signal of the second electrical signal corresponding to the transmitted power of the transmitted beam from the Fabry-Perot interferometer and the first electrical signal corresponding to the transmitted power of the third beam.

[0072] EMBODIMENT 4 a differential amplifier configured to receive the amplitude adjusted first electrical signal and the second electrical signal and generate the error signal in response; 4. The discretely tunable laser system of embodiment 3, further comprising:

[0073] EMBODIMENT 5 the external stabilization circuit further includes a third photodiode optically coupled to the transmitted beam from the Fabry-Perot interferometer and configured to generate a third electrical signal corresponding to the transmitted power of the transmitted beam; The control unit further receiving the third electrical signal; 5. The discretely tunable laser system of embodiment 3 or 4, configured to implement a second PID controller configured to generate a tuning signal for adjusting a cavity length tuning element of the continuously tunable laser based on an RMS value of the third electrical signal and an addition of the error signal.

[0074] EMBODIMENT 6 an isolator optically coupled to the continuously tunable laser and the first tap to allow light to pass in one direction from the continuously tunable laser to the first tap; 6. The discretely tunable laser system of any one of embodiments 1 to 5, further comprising:

[0075] EMBODIMENT 7 7. The discretely tunable laser system of any one of embodiments 1 to 6, wherein the continuously tunable laser is at least one of a laser tuned using a diffraction grating, a laser tuned using an etalon, and a microelectromechanical systems (MEMs) tunable vertical cavity surface emitting laser (VCSEL).

[0076] EMBODIMENT 8 8. The discrete tunable laser system according to any one of embodiments 1 to 7, wherein the Fabry-Perot interferometer is air-spaced.

[0077] EMBODIMENT 9 9. The discretely tunable laser system of any one of the preceding embodiments, wherein the plurality of selectable frequencies includes frequencies in the near infrared band.

[0078] EMBODIMENT 10 9. The discretely tunable laser system of any one of embodiments 1 to 8, wherein the plurality of selectable frequencies includes frequencies between about 272 THz and about 430 THz.

[0079] EMBODIMENT 11 11. The discretely tunable laser system of any one of embodiments 1 to 10, wherein the power of the first beam is greater than the power of the second beam.

[0080] EMBODIMENT 12 In a discretely tunable laser system, a continuously tunable laser configured to emit a beam tunable among a plurality of selectable frequencies, each of the plurality of selectable frequencies being separated in frequency by a plurality of discrete frequency intervals, the plurality of discrete frequency intervals including a maximum frequency interval and a minimum frequency interval, the difference between the maximum frequency interval and the minimum frequency interval being 100 MHz or less; an external stabilization circuit optically coupled to the continuously tunable laser and electrically coupled to a controller, the controller controlling tuning of the continuously tunable laser among the plurality of selectable frequencies; the external stabilization circuit comprising: a first tap configured to split the beam into a first beam and a second beam; a Fabry-Perot interferometer optically coupled to the second beam and configured to generate a plurality of equally spaced resonances in frequency, each of the plurality of resonances defining one of the plurality of selectable frequencies, the Fabry-Perot interferometer generating a transmitted beam and a reflected beam from the second beam; a first photodiode optically coupled to the Fabry-Perot interferometer through a second tap configured to direct the reflected beam from the Fabry-Perot interferometer to a first photodiode, the first photodiode configured to generate a first electrical signal corresponding to a reflected power of the reflected beam reflected by the Fabry-Perot interferometer; a second photodiode optically coupled to the transmitted beam from the Fabry-Perot interferometer, the second photodiode configured to generate a second electrical signal corresponding to a transmitted power of the transmitted beam; and The control unit is generating one or more tuning signals for tuning the continuously tunable laser to one of the plurality of selectable frequencies based on the first electrical signal and the second electrical signal; a discretely tunable laser system configured to transmit the one or more tuning signals to the continuously tunable laser, thereby causing the continuously tunable laser to emit another beam having the one of the plurality of selectable frequencies.

[0081] EMBODIMENT 13 13. The discretely tunable laser system of embodiment 12, wherein the one or more tuning signals adjust one or more tuning elements of the continuously tunable laser, including at least one of a diffraction grating and a laser cavity length, to emit the beam having the one of the plurality of selectable frequencies.

[0082] EMBODIMENT 14 14. The discrete tunable laser system of embodiment 12 or 13, wherein the controller implements a proportional-integral-derivative (PID) controller configured to control the one or more tuning signals by minimizing an error signal, the error signal being the difference between the second electrical signal corresponding to the transmitted power of the transmitted beam and the first electrical signal corresponding to the reflected power of the reflected beam reflected by the Fabry-Perot interferometer.

[0083] EMBODIMENT 15 a differential amplifier configured to receive the first electrical signal and the second electrical signal and, in response, generate the error signal; 15. The discretely tunable laser system of embodiment 14, further comprising:

[0084] EMBODIMENT 16 the external stabilization circuit further includes a third photodiode optically coupled to the transmitted beam from the Fabry-Perot interferometer and configured to generate a third electrical signal corresponding to the transmitted power of the transmitted beam; The control unit further receiving the third electrical signal; 16. The discretely tunable laser system of embodiment 14 or 15, configured to implement a second PID controller configured to generate a tuning signal for adjusting a cavity length tuning element of the continuously tunable laser based on an RMS value of the third electrical signal and an addition of the error signal.

[0085] EMBODIMENT 17 an isolator optically coupled to the continuously tunable laser and the first tap to allow light to pass in one direction from the continuously tunable laser to the first tap; 17. The discretely tunable laser system of any one of embodiments 12 to 16, further comprising:

[0086] EMBODIMENT 18 18. The discretely tunable laser system of any one of embodiments 12 to 17, wherein the continuously tunable laser is at least one of a laser tuned using a diffraction grating, a laser tuned using an etalon, and a microelectromechanical systems (MEMs) tunable vertical cavity surface emitting laser (VCSEL).

[0087] EMBODIMENT 19 19. The discrete tunable laser system according to any one of embodiments 12 to 18, wherein the Fabry-Perot interferometer is air-spaced.

[0088] EMBODIMENT 20 20. The discrete tunable laser system of any one of embodiments 12 to 19, wherein the plurality of selectable frequencies includes frequencies between about 272 THz and about 430 THz. [Explanation of symbols]

[0089] 100 Continuously Tunable Laser 200, 202 Discretely tunable laser system 210 Control section 230A 1st Tap 230B 2nd Tap 230C 3rd Tap 240A First Photodiode 240B Second Photodiode 250 Fabry-Perot Interferometer

Claims

1. In a discretely tunable laser system, a continuously tunable laser configured to emit a beam tunable among a plurality of selectable frequencies, each of the plurality of selectable frequencies being separated in frequency by a plurality of discrete frequency intervals, the plurality of discrete frequency intervals including a maximum frequency interval and a minimum frequency interval, the difference between the maximum frequency interval and the minimum frequency interval being 100 MHz or less; an external stabilization circuit optically coupled to the continuously tunable laser and electrically coupled to a controller, the controller controlling tuning of the continuously tunable laser among the plurality of selectable frequencies; the external stabilization circuit comprising: a first tap configured to split the beam into a first beam and a second beam; a second tap configured to split the second beam into a third beam and a fourth beam; a first photodiode configured to generate a first electrical signal corresponding to a transmitted power of the third beam; a Fabry-Perot interferometer configured to generate a plurality of resonances upon incidence of the fourth beam, the plurality of resonances being equally spaced in frequency, each of the plurality of resonances defining one of the plurality of selectable frequencies, and generating a transmitted beam from the fourth beam; and a second photodiode configured to generate a second electrical signal corresponding to a transmitted power of the transmitted beam from the Fabry-Perot interferometer; and The control unit is generating one or more tuning signals for tuning the continuously tunable laser to one of the plurality of selectable frequencies based on the first electrical signal and the second electrical signal; a plurality of selectable frequencies that are each selected from a plurality of selectable frequencies, each of which is configured to transmit one or more tuning signals to the continuously tunable laser, thereby causing the continuously tunable laser to emit another beam having the one of the plurality of selectable frequencies.

2. 2. The discretely tunable laser system of claim 1 , wherein the one or more tuning signals adjust one or more tuning elements of the continuously tunable laser, including at least one of a diffraction grating and a laser cavity length, to emit the beam having the one of the plurality of selectable frequencies.

3. 2. The discretely tunable laser system of claim 1, wherein the controller implements a proportional-integral-derivative (PID) controller configured to control the one or more tuning signals by minimizing an error signal, the error signal being a difference between an amplitude-adjusted first electrical signal of the second electrical signal corresponding to the transmitted power of the transmitted beam from the Fabry-Perot interferometer and the first electrical signal corresponding to the transmitted power of the third beam.

4. a differential amplifier configured to receive the amplitude adjusted first electrical signal and the second electrical signal and generate the error signal in response; The discretely tunable laser system of claim 3 further comprising:

5. the external stabilization circuit further includes a third photodiode optically coupled to the transmitted beam from the Fabry-Perot interferometer and configured to generate a third electrical signal corresponding to the transmitted power of the transmitted beam; The control unit further receiving the third electrical signal; 4. The discretely tunable laser system of claim 3 configured to implement a second PID controller configured to generate a tuning signal for adjusting a cavity length tuning element of the continuously tunable laser based on an RMS value of the third electrical signal and an addition of the error signal.

6. an isolator optically coupled to the continuously tunable laser and the first tap to allow light to pass in one direction from the continuously tunable laser to the first tap; The discretely tunable laser system of claim 1 further comprising:

7. 6. The discretely tunable laser system of claim 1, wherein the continuously tunable laser is at least one of a grating tuned laser, an etalon tuned laser, and a microelectromechanical systems (MEMs) tunable vertical cavity surface emitting laser (VCSEL).

8. 6. The discretely tunable laser system of claim 1, wherein the Fabry-Perot interferometer is air-spaced.

9. 6. The discretely tunable laser system of claim 1, wherein the plurality of selectable frequencies includes frequencies in the near infrared band.

10. 6. The discretely tunable laser system of claim 1, wherein the plurality of selectable frequencies includes frequencies between about 272 THz and about 430 THz.

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