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  • H01S3/105—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
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  • H01S3/1396—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length by using two modes present, e.g. Zeeman splitting
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• • G—PHYSICS
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  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
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  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
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Definitions

• Provisional Patent Application 61 /676712 is hereby incorporated by reference.
• the present invention relates to tunable lasers, widely tunable lasers, wavelength swept sources, amplified tunable lasers, rapidly tuned lasers, and optical systems enabled by these devices.
• Widely and rapidly tunable lasers are important for a variety of detection, communication, measurement, therapeutic, sample modification, and imaging systems.
• swept source optical coherence tomography (SSOCT) systems employ repetitively swept tunable lasers to generate subsurface microstructural images of a wide range of materials.
• SSOCT swept source optical coherence tomography
• wide tuning range translates to higher axial measurement resolution, and higher tuning speed enables real-time acquisition of large data sets.
• variable tuning speed enables trading off imaging range and resolution as required for different applications.
• long coherence length which is equivalent to narrow linewidth, enables long imaging range.
• transient gas spectroscopy as, for example, described in (Stein, B.A., Jayaraman, V. Jiang, J.J, et al, "Doppler-limited H20 and HF absorption spectroscopy by sweeping the 1321 -1354 nm range at 55kHz repetition rate using a single-mode MEMS-tunable VCSEL," Applied Physics B: Lasers and Optics 108(4), 721 -5 (2012)).
• tuning speed enables characterization of time-varying processes, such as in engine thermometry.
• Narrow spectral width enables resolution of narrow absorption features, such as those that occur at low gas
• tuning speed enables imaging of time- varying physiological processes, as well as real-time volumetric imaging of larger data sets.
• variability of tuning speed enables switching between high speed, high resolution short-range imaging, and low speed, low resolution long range imaging in a single device, which is of great utility in, for example, ophthalmic imaging, as described in (Grulkowski, I, Liu, J. J., Potsaid, B.
• Further desirable properties of widely tunable lasers include high output power, center wavelength flexibility, spectrally shaped output, monolithic and low-cost fabrication, and compatibility with array technology.
• High power increases signal to noise ratio for virtually every application.
• Center wavelength flexibility translates into greater utility in a larger variety of applications.
• Spectrally shaped output also increases signal to noise ratio and improves thermal management.
• Monolithic, low cost fabrication has obvious advantages, and array technology simplifies applications in which multiple sources are multiplexed.
• FDML and ECTL devices are essential ly multi-longitudinal mode devices, which sweep a cluster of modes instead of a single mode across a tuning range. This results in limited imaging range for SSOCT and limited spectral resolution for spectroscopic applications.
• Both FDML and ECTL are also non-monolithic sources, which are assembled from discrete components, and therefore not low cost devices or compatible with array fabrication.
• the ECTL further suffers from fundamental speed limitations of about 100kHz repetition rate or less, due to the long time delay in the external cavity, as described in (Huber, R., Wojtkowski, M., Taira, K.
• the SGDBR is a single transverse and longitudinal mode device, and has the potential for long imaging range and narrow spectral width. Tuning, however, is accomplished by discontinuous hopping amongst various modes, which tends to
• the mode-hopping also requires multiple timing electrodes, complicated drive circuitry and associated speed limitations.
• the SGDBR also suffers from limited tuning range relative to external cavity and FDML lasers, since the latter use lossless tuning mechanicms, while the SGDBR is tuned by free carrier injection, which introduces free carrier losses and limits tuning range.
• the SGDBR also suffers from center wavelength inflexibility, due to the need for complex regrowth fabrication technology which is only mature in the Indium Phosphide material system.
• MEMS-tunable vertical cavity lasers offer a potential solution to the problems above.
• the short cavity of MEMS-VCSELs leads to a large longitudinal mode spacing and relative immunity to mode hops.
• the MEMS-VCSEL requires only one tuning electrode to sweep a single mode across the tuning range, and therefore offers the promise of long SS-OCT imaging range with minimal measurement artifacts, and rapid tuning.
• the short cavity and the short mass of the MEM S mirror offer the potential for very high speed.
• MEMS-VCSEL technology can also be extended to a large variety of wavelength ranges difficult to access with many other types of sources, making them appropriate for other types of spectroscopic, diagnostic, and detection systems.
• MEMS-VCSELs have the potential for wide tuning range, as discussed in US patent 7468997. Until 2011 , however, the widest MEMS-VCSEL tuning range achieved was 65 nm around 1550 nm, as described in (Matsui, Y., Vakhshoori, D., Peidong, W. et ⁇ , "Complete polarization mode control of long- wavelength tunable vertical-cavity surface-emitting lasers over 65-nm tuning, up to 14-mW output power," IEEE Journal of Quantum Electronics, 39(9), 1037-10481048 (2003).
• the MEMS-VCSEL described by Jayaraman, et al. in 2011 represented a major innovation in widely tunable short, cavity lasers. Achieving peformance and reliabiity appropriate for commercial optical systems, however, requires optimization of tuning speed, frequency response of tuning, tuning range, spectral shape of tuning curve, output power vs. wavelength, post-amplified performance, gain and mirror designs, and overall cavity design. Numerous design innovations are required to improve upon the prior art to achieve performance and reliability necessary for these commercial systems.
• This document provides several preferred embodiments of a tunable source comprising a short-cavity laser optimized for performance and reliability in SSOCT imaging systems, spectroscopic detection systems, and other types of detection and sensing systems.
• This document presents a short cavity laser with a large free spectral range cavity, fast tuning response and single transverse, longitudinal and polarization mode operation.
• the disclosure includes embodiments for fast and wide tuning, and optimized spectral shaping.
• Preferred embodiments include both electrical and optical pumping in a MEMS-VCSEL geometry with mirror and gain regions optimized for wide tuning, hi gh output power, and a variety of preferred wavelength ranges.
• Other preferred embodiments include a semiconductor optical amplifier, combined with the short-cavity laser to produce high-power, spectrally shaped operation.
• Several prefen-ed imaging and detection system embodiments make use of this tunable source for optimized operation.
• One embodiment provides a tunable laser operative to emit tunable radiation over an emission wavelength range having a center wavelength, with an output power spectrum over said wavelength range and an average emission power, said tunable laser comprising: an optical cavity including a first and second mirror; a quantum well gain region mterposed between said first and second mirrors and comprising at least one quantum wel l; a tuning region; and means for adjusting an optical path length of said tuning region; wherein: a free spectral range (FS ) of said optical cavity exceeds 5% of said center wavelength; said tunable laser operates substantially in a single longitudinal and transverse mode over said wavelength range; said means for adjusting an optical path length has a wavelength tuning frequency response with a 6-dB bandwidth greater than about 1 kHz, and each of said at least at least one quantum well is substantially aligned with a peak in an optical standing wave pattern of said optical cavity.
• FS free spectral range
• Figure 1 illustrates an embodiment of widely tunable short cavity laser according to an embodiment.
• Figure 2 illustrates an output power spectrum of a widely tunable short- cavity laser.
• Figure 3 illustrates the definition of free spectral range.
• Figure 4 shows a water vapor absorption spectrum in the 1330-1365 nm rang.
• Figure 5 shows a measurement dynamic coherence length obtained by the rolloff of the OCT point spread function vs. imaging depth
• Figure 6 illustrates an embodiment of a widely tunable short-cavity laser with closed loop control.
• Figure 7 illustrates a MEMS-VCSEL implementation of a tunable short cavity laser operating near 1310 am.
• Figure 8 illustrates an axial refractive index profile of a short cavity laser having 4 standing wave maxima between two mirrors of the cavity.
• Figure 9 illustrates the static and dynamic tuning response of the MEMS- VCSEL illustrated in Fig. 7.
• Figure 16 illustrates an ASE spectrum from a dual-quantum state semiconductor optical amplifier.
• Figure 17 illustrates a widely tunable short cavity laser coupled to an optical amplifier, the output of which is coupled to a synchronously tuned optical filter.
• Figure 21 illustrates an amplified and pre-amplified spectrum of a widely tunable short cavity laser.
• Figure 22 illustrates various output power spectra of a widely tunable short-cavity laser operating near 1310 nm.
• Figure 23 illustrates a MEMS-V CSEL implementation of a widely tunable short-cavity laser operating near 1060 nm.
• Figure 25 illustrates steps 1-4 in the fabrication of a widely tunable short cavity laser realized as a MEMS-VCSEL.
• Figure 26 illustrates steps 5-6 in the fabrication of a widely tunable short cavity laser realized MEMS-VCSEL
• an anti-reflection coating is placed between the gain region and the tuning region to suppress reflections in the device and extend the tuning range.
• This anti-reflection coating can be a quarter wavelength of material such as silicon nitride or silicon oxynitride, in the preferred case when the timing region is air, and the gain region is semiconductor.
• Coherence length can be adjusted by adding a noise waveform to the tuning region, or otherwise amplitude or phase modulating the source.
• External means could include, for example, a temporal
• the frequency response of the optical path length of a timing region to an applied tuning signal has a 6-dB bandwidth that exceeds about 1 kHz. Normally, this 6-dB bandwidth starts at DC but can start at some non-zero frequency as well. The 1 kHz bandwidth distinguishes the present invention from other types of timing mechanisms employed in the prior art, such as electro-thermal tuning in (Gierl, C, Gruendl, T., Debehap, P.
• the measurement of weak spectroscopic signals could require slow scanning speeds, whereas strong spectroscopic signals could be monitored such that dynamic temporal effects could be captured.
• Many applications in SSOCT could also benefit from variable scan frequency, which enabl es tradeoff of imaging resolution and imaging range with imaging speed.
• the tuning region can be driven by a non-repetitive waveform, in response to an external trigger, or by any repetitive or no -repetitive arbitrary waveform. Examples of this are in transient spectroscopy, where it is advantageous to measure the transmission, absorption, or reflection spectrum of a material shortly after an event, such as an explosion, chemical reaction, or a biological event. Non-repetitive scanning would also facilitate new modes of operation whereby a number of narrow regions of interest separated by large regions of no interest could be interrogated with the laser in an optimized manner.
• One example is a series of slow scans across narrow spectroscopic features that are separated by large regions wherein the large regions are scanned at high speed.
• many new operating modes are made possible by the extremely low mass of the tuning element that allows for rapid acceleration and deceleration of the laser tuning speed.
• a portion of the light emitted from the tunable short- cavity laser is split to a wavelength-sensing element 610, which can comprise elements such as a prism, grating, optical filter, or optical interferometer.
• a dispersive element like a prism or a grating
• a position-sensing element like a detector array would be combined with the dispersive element to detect diffracted or refracted angle and infer wavelength offset from the desired position and feed this error signal to the tuning drive waveform 620.
• the DBR is preferred for both mirrors, although a high contrast grating as used by prior art lasers can also be employed, as described in for example (Chase, C, Rao, Y., Hofrnann, W. et al., "1550 nm high contrast grating VCSEL," Optics Express, 18(15), 15461-15466 (2010)).
• the bottom mirror 710 of Fig. 7, corresponding to the second mirror 140 of Fig. 1 is comprised of alternating quarter wave layers of GaAs and Aluminum oxide (AlxOy).
• This type of mirror is formed by lateral oxidation of an epitaxialiy grown stack of GaAs/AlAs, as described in ( acDougal, M H., Dapkus, P. D., Bond, A. E. et al., "Design and fabrication of VCSELs with Al xO y-GaAs DBRs," IEEE Journal of Selected Topics in Quantum Electronics, 3(3), 905-915915 (1997)).
• the GaAs/AlxOy mirror has a large reflectivity and wide bandwidth with a small number of mirror periods.
• the preferred number of mirror periods for the back mirror, when light is coupled out the top mirror as in Fig. 7, is six or seven periods, creating a theoretical lossless reflectivity of >99.9%.
• Other implementations of this mirror could use AlGaAs/ AlxOy, where the aluminum content of the AlGaAs is less than about 92%, so that it does not oxidize appreciably during lateral oxidation of the AlAs to form AlxOy.
• Use of AlGaAs instead of GaAs for the low index material is advantageous for increasing the bandgap of the low-index material to make it non-absorbing at the lasing wavelength or at the pump wavelength if the laser is optically pumped.
• the theoretical lossless reflectivity exceeds 99,5% over a range of at least 10% of the center wavelength, as can be calculated by those skilled in the art of mirror design.
• Fig. 7 uses M EMS actuation to control the thickness of an airgap tuning region to control the output wavelength of the device in the range of 1310 nm.
• Application of a voltage between the actuator contacts 730, 740 shown contracts the airgap and tunes the laser to shorter wavelengths.
• the MEMS structure shown consists of a rigid supporting structure 750 and a suspended deformable dielectric membrane 760, on which is the suspended top mirror 720.
• the top of the dielectric membrane 760 is metallized to enable electrostatic force to be applied by the actuator contacts 730, 740.
• the membrane itself is transparent, runs underneath and is integral with the suspended mirror, and contributes constructively to the reflectivity of the suspended mirror.
• the membrane thickness is an odd number of quarter wavelengths at the center wavelength of the emitted tuned radiation. For many wavelengths of interest, such as in the 600-2500 nm range, the ideal thickness is about 3 ⁇ 4 wavelength.
• the InP-based MQW region must be joined to the GaAs-based fully oxidized mrror through a wafer bonding process, as described in fixed wavelength 1310 imi VCSELs such as in
• the multi-quantum well region is preferably comprised of multiple compressively strained AlIiiGaAs quantum wells, with strain in a range of 1-1.5%.
• the A!lnGaAs quantum well is however higher gain and more wavelength flexible, and is therefore preferred.
• Figs. 25 and 26 illustrate the major steps of a fabrication sequence used to fabricate the preferred implementation of the 1310 nm tunable short cavity laser in Fig. 7. Processing of devices in a wavelength range of 650-2300 nm can proceed in a similar fashion, with the except that GaAs-based devices do not require the first wafer bonding step shown in Fig. 25, since mirror and gain region can be epitaxially grown in one step.
• the first step 2510 involves wafer bonding of the MQW region epitaxially grown on an InP substrate to a GaAs/AiAs mirror structure epitaxially grown on a GaAs substrate.
• One further advantage of periodic gain is that the wide spacing between quantum wells prevents strain accumulation and reduces the need for strain compensation.
• the ideal pump wavelength for the 1310 nm tunable VCSEL shown is in a range of about 850-1050 nm.
• three quantum we! is can be placed on three separated standing wave peaks, and the region between them can be made of A!InGaAs substantially lattice-matched to InP, and of a composition that absorbs incoming pump radiation.
• the gain region is separated from the absorbing regions, and photo-generated carriers in the absorbing regions diffuse into gain region.
• Fig. 8 represents the fully oxdized mirror and the periodic structure at the right of Fig. 8 represents the suspended dielectric mirror including the thicker first layer which is the silicon nitride membrane.
• the MQW gain region and airgap tuning region between the mirrors are also indicated in Fig. 8.
• InGaAs these include but are not limited to Al lnGaP, AlInGaAs, InGaAsP, IiiGaP, AlGaAs, and GaAs.
• GaAs quantum wells would be used in about the 800-870 nm range, AlGaAs wells in about the 730-800 nm range, AllnGaP and InGaP in about the 600-730 nm range, and InGaAsP or AlInGaAs as alternative materials in about the 8G0-900nm range.
• the wavelength range of 700-1100 nm is of particular interest in SSOCT ophthalmic imaging and also oxygen sensing, and the range of about 990-1110 nm is of greatest interest for ophthalmology.
• periodic gain can be empl oyed in the structure of Fig, 23.
• a periodic gain structure with three InGaAs quantum wells 2320 at three standing wave peaks in the cavity, separated by GaAs barriers which absorb the pump radiation and generate electrons and holes which diffuse into the quantum wells.
• Typical quantum well widths are 6-12 nm and typical Indium percentage is about 20%.
• Quantum well widths greater than about 8 nm lead to a second confined quantum state and broadened gain.
• a structure using this approach generated the tuning results shown in Fig. 24, illustrating a tuning range of 100 nm around 1060 nm.
• the FS of this structure is around 100 nm or about 9.4% of the center wavelength.
• FSR can be increased to >10% as in the 1310 nm structure by placing ail quantum wells on a single standing wave peak or by placing four quantum wells on two standing wave peaks.
• strain compensation of the InGaAs with tensile-strained GaAsP as described in the prior art on fixed wavelength VCSELs (Hatakeyama, H., Anan, T., Akagawa, T, et al, "Highly Reliable High-Speed 1.1 -mu m-Range VCSELs With InGaAs/GaAsP-MQWs," IEEE Journal of Quantum Electronics . , 46(6), 890-897 (2010)) can be employed.
• a tunable short- cavity l aser operating in th e 400- 550 nm range could be realized using materials grown on GaN substrates as described by researchers making fixed wavelength VCSELs (Higuehi, Y., Omae, K., Matsumura, H. et al, "Room-Temperature CW Easing of a GaN-Based Vertical-Cavity Surface-Emitting Laser by Current Injection," Applied Physics Express, 1(12), (2008)).
• Implementation of embodiments of the present invention in the vi sibl e range of 400-700 nm range has application in optical metrology tools and biological and medical spectroscopy.
• Fig, 14 illustrates an example top mirror designed reflectivity for a tunable short-cavity laser configured to emit in the range of 1200-1400 nm, with an optical pump at 1050 nm.
• the top mirror can be made to have minimal reflectivity 1410 at the pump wavelength at 1050 nm, while having high reflectivity 1420 at the desired 1200-1400 nm emission wavelength range,
• Another way of changing the spectral shape is to control the pump energy into the gain region dynamically during wavelength tuning. In the case of an optically pumped device, this can be controlling the pump energy into the device, and in the case of an electrically pumped device the drive current would be controlled. Shaping of the pump energy can also improve thermal management of the device.
• FIG. 12 shows pictures of a sampling of actuator geometries resulting in the frequency responses of Fig. 10.
• Fig. 12 top-view pictures of several MEMS tunable VCSEL structures having four or eight supporting struts 1210.
• Fig. 10 shows the tuning of a MEMS-VCSEL wavelength as a function of drive frequency applied to the MEMS- actuated airgap tuning mechanism.
• the resonant frequencies are in a range of about 200 kHz to about 500 kHz, and the 6 dB bandwidths of the fastest devices are approaching 1 MHz.
• [ ⁇ 85] Also shown is a variation in the damping of the actuator, manifested by varying amounts of peaking at resonance.
• the damping is primarily caused by squeeze- film damping, which represents interaction with viscous air. As the actuator area is increased or the airgap is reduced, the squeeze-film damping goes up, flattening the frequency response. A flat wide frequency response is desirable for variable speed drive, and for linearization of drive through multiple harmonics. Though damping through squeezed film effects is demonstrated in Fig. 10 in a MEMS device, similar effects can be seen in other airgap tuned devices such as piezo-driven devices.
• the damping of the MEMS actuator through a variety of methods, including changing the actuator area or shape to change interaction with viscous air, changing the background gas composition or gas pressure, which further changes the contribution of squeeze-film damping, changing the airgap thickness, and changing the size of holes or perforations in the actuator to change the regime of fluid flow through the holes from a turbulent to a non-turbulent regime. Additionally, annealing the actuator can change the stress of various materials in the actuator, which will, have an effect on damping.
• the frequency responses represented by Fig. 10 are representative and not limiting.
• the resonance frequency can be increased by stiffening the membrane through increased tensile stress, increased thickness (for example 5/4 wavelength), reduced suspended mirror diameter and thickness, or shortened arms, such that 6-dB bandwidths in excess of 2 MHz can be achieved, as can be calculated by those skilled in the art of finite element modeling.
• resonant frequency can be decreased well below 100 kHz by changing the same parameters in the opposite direction.
• other geometries are possible, such as a spiral arm geometry, which reduces resonant frequency, or a perforated membrane without clearly delineated supporting struts.
• the silicon nitride membrane discussed above is highly insulating, and may therefore be prone to charging and electrostatic drift. Introducing a small amount of electrical conductivity in the membrane can reduce the propensity to charging. For silicon nitride, this electrical conductivity can be introduced by using a non- stoichiometric silicon-rich film, or by doping the silicon nitride film with silicon.
• ripple of a particular spectral period having an amplitude of 1% or more relative to an average power can manifest itself as a spurious reflector at an apparent distance in an SSOCT image.
• Ripple is typically caused by spurious reflections outside the laser cavity. These reflections can come from coupling lenses or other optical elements in the optical system, or they can come from substrate reflections in a vertical cavity laser. For example, in the laser of Fig. 7, reflections coming from below the second mirror, such as from the bottom of the GaAs substrate 770 on which this devi ce is disposed, can cause ripple.
• the substrate reflection amplitude can be suppressed by various means, including but not limited to increasing the reflectivity of the second mirror, introducing loss through dopants in the substrate, increasing substrate thickness, or roughening the backside of the substrate to increase scattering.
• An optimal grit for substrate roughening to increase scattering is a grit size >30 um in the range of 900-1400 urn tunable emission.
• use of a fully oxidized bottom mirror having 7 or more periods, which has a theoretical lossless reflectivity of >99.5%, can suppress ripple to ⁇ 1 % levels.
• Another important performance feature of an embodiment of the present invention is operation in a fixed polarization state throughout a tuning range of the wavelength swept emission.
• Operation in a single polarization state is important if operating with any polarization- sensitive components in the optical system, such as polarization-selective optical amplifiers.
• Such systems may also employ the polarization stable device according to an embodiment of the present invention in combination with polarization maintaining fiber. Polarization switching over the emission wavelength range can cause power dropouts or image artifacts in an SS-OCT system, and compromise dynamic coherence length.
• Operation in a single polarization state throughout a tuning range of the device can be accomplished in a variety of ways.
• One way is to introduce one or more nanowires integral with the optical cavity of the device. With respect to Fig. 7, this nanowire can be disposed on top of the MQW gain region 780 adjacent the tunable airgap, in the center of the optical path. Alternatively it could be placed on top of the suspended mirror.
• a nanowire is an element which can cause polarization-dependent scattering or absorption of light. Typical dimensions might be 50 nm wide, several microns long, and 10 nm thick.
• the nanowire might be constructed of metal or may simply be a refractive index perturbation.
• the tunable short-cavity laser described here can be combined in array- form to generate an aggregate tunable laser source with enhanced optical properties.
• the laser is a MEMS-tunable vertical cavity laser
• the array can be fabricated in monolithic form.
• a first tunable short cavity laser TCSL 1 and a second tunable short-cavity laser TCSL 2 are multiplexed on to a common optical path, using a beam splitter, fiber coupler or other know combining element 2810.
• Each TCSL is driven to have a bidirectional tuning over its tuning range, as shown by the solid wavelength trajectory 2820 in Fig. 28C for TCSL 1 and the dashed trajectory 2830 in Fig. 28C for TCSL 2.
• Each laser is repetitively scanned at a repetition period T, but the scan of TCSL 2 is time-delayed relative to that of TCSL 1 by half the repetition period.
• the pump energy 2840, 2850 (either electrical or optical pump) for each of the two TCSLs is turned off during the backward wavelength scan such that only the forward or front of half of the wavelength scan, when pump energy is non-zero, emits laser radiation, in some instances, if the FSR is much larger than the gain bandwidth of the supporting material, scanning the t ning element beyond the material gain bandwidth will automatically shut off the laser without having to turn off the pump energy.
• Fig. 28D The wavelength trajectory of the multiplexed output is shown in Fig. 28D, comprising components from both TCSL I (solid) 2860 and TCSL 2 (dashed) 2870, and illustrating unidirectional scanning at a new repetition period T/2 which is half the original period T of each TCSL.
• T/2 the original period T of each TCSL.
• the sweep rate has been multiplied by a factor of two.
• the same principle could be applied to N lasers and multiplication of the sweep rate by a factor of N.
• interleaving TCSLs can also be used for more than multiplying sweep rate, but also for multiplying tuning range, interleaving different tuning ranges, tuning speeds, or tuning trajectories, or a for a variety of other purposes evident to those skil led in the art of SSOCT, spectroscopy, communications, or optical detection.
• the tunable short-cavity laser described thus far can be combined with an optical amplifier to create an amplified tunable source with increased output power and other advantageous properties for imaging.
• the amplifier can be a semiconductor amplifier, a fiber amplifier such as a praseodymium-doped fiber amplifier for operation in a window around 1300 nm, an Ytterbium-doped amplifier for operation in a window around 1050 nm , a Fluoride-doped extended bandwidth fiber amplifier near 1050 nm, or any kind of optical amplifier.
• the use of an amplifier can also enable the interleaving scheme above, wherein a high extinction ratio optical amplifier can be used to turn on one source at the appropriate time, instead of turning off the pump energy to that source.
• a tunable short cavity laser 1510 according an embodiment of the present invention emits an input tunable radiation 1520 directed to an input side of the optical amplifier 1530.
• This input tunable radiation has an input average power, input power spectrum, input wavelength range, and input center wavelength.
• the amplifier amplifies the input tunable radiation to generation an output tunable radiation having an output average po was, output center wavelength, output wavelength range, and output power spectrum.
• the amplifier is operated in a saturated regime, as is well-known to those skilled in the art of optical amplifiers.
• the saturated regime can suppress noise fluctuations present in the input tunable radiation, and can also provide advantageous spectral shaping in which a full-width at half-maximum (FWHM) of the output tunable radiation can exceed a FWHM of the output tunable radiation.
• FWHM full-width at half-maximum
• An example of this is shown in Fig. 21, in which the amplified tunable spectrum 2110 has a wider FWHM than the input tunable radiation 2120 from the tunable short cavity laser.
• the semiconductor optical amplifier can be configured to be polarization sensitive, by using all cornpressively strained or tensile-strained quantum wells, or polarization insensitive by using both types of strain in a single structure to provide gain at all polarizations.
• the center wavelength of the input tunable radiation is at a longer wavelength than a center wavelength of amplified spontaneous emission (ASE) emitted by the amplifier.
• ASE amplified spontaneous emission
• the amplifier ASE is typically blue-shifted relative to the amplifier gain spectrum, so this configuration brings the spectrum of input tunable radiation into more optimal alignment with the amplifier gain spectrum.
• varying the alignment of the amplifier ASE relative to the input power spectrum can provide advantageous spectral shaping.
• Fig. 15 The basic configuration of Fig. 15 can be augmented with various forms of filtering to create a lower noise amplified swept source. Many swept source laser application in metrology, spectroscopy, and biophotonics would benefit from the suppression of broadband ASE, and an improvement in side mode suppression.
• the addition of an additional tunable spectral filter to the system, either internal to the laser cavity, between the laser and amplifier, or at the output of the system is one means of providing improved performance in this regard.
• the amplifier shown in Fig. 15 can be a tunable resonant amplifier, such as a vertical cavity amplifier described by (Cole, G. D., Bjorlin, E. 8., Chen, Q.
• FIG. 17 A number of other preferred configurations are illustrated by Figs. 17-20.
• a synchronously tuned optical filter 1710 whose passband is aligned at all times with the wavelength of the input tunable radiation, is placed after the broadband optical amplifier 1720 to reduce residual ASE noise and improve a signal to noise ratio of the amplified tunable radiation.
• the same synchronous!)' tuned optical filter 1810 is placed between the tunable short cavity laser 1830 and the optical amplifier 1820, to improved a side-mode suppression of the input tunable radiation prior to amplification.
• FIG. 19 Another configuration is illustrated in Fig. 19, where two amplification stages 1910, 1920 are used. These can be implement as two separate amplifiers, or as a single waveguide amplifier with split amplifier contacts.
• the use of two amplification stages 1910, 1920 provides further flexibility in spectral shaping. For example, the gain spectrum of the two amplifiers can be shifted relative to each other, either by biasing identical epitaxial structures differently, or by using different epitaxial structures in the two amplifiers.
• the use of two amplification stages can also create higher gain and greater output power.
• the basic configuration of the tunable short-cavity laser in combination with an amplifier can be realized with semiconductor optical amplifiers employing a variety of materials appropriate for a vari ety of wavelength ranges.
• the amplifier can operate in the 1200-1400 nm range appropriate for SSOCT and water vapor spectroscopy. In this range, use of an AlInGaAs or InGaAsP quantum well on InP produces the required gain.
• the amplifier can operate in about the 800-1 100 nm range appropriate for ophthalmic SSOCT, employing at least one compressively strained InGaAs quantum well.
• a system for SSOCT can employ a tunable laser comprising the tunable short-cavity laser described above, in combination with a means for splitting tunable radiation from the tunable laser to a reference path and a sample path, and an optical detector configured to detect an interference signal between light reflected from said sample and traversing said reference path. Signal processing of this interference signal can then be used to reconstruct structural or compositional information about he sample, as is well-know to those skil led in the art of SSOCT.
• the tunable short cavity laser described can, in combination with a dispersive optical element, be employed in a system for optical beam steering.
• a dispersive optical element For example, it is well-known that the diffraction angle of a grating is a function of the wavelength of input tunable radiation. Thus, tuning the radiation will scan the diffraction angle and achieve optical beam steering.
• Other dispersive elements such as prisms can also be employed.
• An embodiment of the present invention can al so be used to create a tunable oscillator, by beating the tunable output of the short-cavity laser with a fixed wavelength reference laser.
• This beating can be accomplished by, for example, an optical detector that responds to incident optical power. If two collinear laser beams impmge on this detector, the detector output will oscillate at the difference in optical frequencies between the two laser beams, provided that difference frequency is within the detector bandwidth. As one laser is tuned, this difference frequency will also time, creating a tunable oscillator down-shifted from optical frequencies to lower frequencies.

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