Cat-eye swept-source laser for OCT and spectroscopy

Abstract

A tunable or swept laser architecture is provided that is also suitable for other applications such as swept-source optical coherence tomography and spectroscopy using a cat's eye configuration with a tilt-tuned interference thin film filter (preferably a transmissive tilt-tuned interference thin film filter). [Solution] The tunable laser 100 comprises a gain chip 110 that amplifies light in a laser cavity, a collimating lens 118 that collimates light from the gain chip 110, an end reflector 122 of the laser cavity, a focusing lens 120 that focuses the collimated light 124 onto the end reflector 122, a thin-film bandpass filter 130 between the collimating lens 118 and the focusing lens 120, and at least one angle control actuator 132 that changes the angle of the thin-film filter 130 relative to the collimated light 124.

Description

Related Applications

[0001] This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 63 / 319,973, filed March 15, 2022, the entire contents of which are incorporated herein by reference. [Technical field]

[0002] Optical coherence tomography (OCT) is a non-invasive cross-sectional imaging modality used in many medical imaging disciplines. For example, in ophthalmology, OCT is widely used to image the retina, choroid, and anterior segment of the eye. It also allows for functional imaging of blood flow velocity and the microvasculature. [Background technology]

[0003] Fourier domain OCT (FD-OCT) has recently attracted attention due to its high sensitivity and high imaging speed compared to time domain OCT (TD-OCT), which uses an optical delay line to perform mechanical depth scanning at a relatively slow imaging speed. FD-OCT spectral information is discerned by using a dispersive spectrometer in the detection arm (spectral domain OCT (SD-OCT)) or by rapidly scanning a swept laser source (swept-source OCT (SS-OCT)).

[0004] Swept-source OCT (SS-OCT) has several advantages over spectrometer-based FD-OCT, including robustness against motion artifacts and fringe washout, low sensitivity roll-off, and high detection efficiency.

[0005] Many different approaches have been taken to develop high-speed swept sources for SS-OCT. Some approaches use semiconductor optical amplifier (SOA)-based ring laser structures (see, for example, Yun et al. “High-speed optical frequency-domain imaging” Opt. Express 11:2953 2003 (Non-Patent Document 1), Huber et al. “Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines / s,” Opt. Express 13, 3513 2005 (Non-Patent Document 2) and others). Other examples include short-cavity lasers (see, for example, Kuznetsov et al. “Compact Ultrafast Reflective Fabry-Perot Tunable Lasers For OCT Imaging Applications,” Proc. SPIE 7554:75541F 2010 (Non-Patent Document 3)). SOA-based ring laser structures are essentially limited to wavelength sweeping in the positive direction (increasing the wavelength) because of the significant power loss that occurs in tuning in the negative direction. This is because four-wave mixing (FWM) in the SOA causes a negative frequency shift in the light in the cavity as it propagates through the SOA (Bilenca et al “Numerical study of wavelength-swept semiconductor ring lasers: the role of refractive-index nonlinearities in semiconductor optical amplifiers and implications for biomedical imaging applications,” Opt. Lett. 31: 760-762 2006 (Non-Patent Document 4)).

[0006] Commercially available short-cavity lasers exceeding 100 kHz (Axsun Technologies, Billerica, MA) have been reported (see, for example, Kuznetsov et al “Compact Ultrafast Reflective Fabry-Perot Tunable Lasers for OCT Imaging Applications,” Proc. SPIE 7554: 75541F 2010). Short-cavity lasers allow for much faster sweep speeds than traditional swept-laser techniques, because spontaneous emission noise can build up lasing and saturate the gain medium much faster (R. Huber et al “Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines / s,” Opt. Express 13: 3513). 2005 (Non-Patent Document 6). However, the effective duty ratio of a bidirectionally swept short-cavity laser is limited to less than 50% due to the above-mentioned FWM effect, which limits the effective repetition rate of the laser.

[0007] More recently, tunable vertical-cavity surface-emitting lasers (VCSELs) have emerged from Thorlabs and Axsun Technologies, which imply shorter cavities, allowing even faster sweeps.

[0008] Other techniques proposed to increase the effective repetition rate of SS-OCT systems include sweep buffering using delay lines and multiplexing multiple light sources to improve the laser duty cycle. Such sweep wave multiplexing techniques include the inclusion of components that introduce orthogonal polarizations to the sweep waves from each optical path. Combining the different polarizations with a polarizing beam splitter provides a very optically efficient way to send light into a single beam path.

[0009] Goldberg et al. demonstrated that a ping-pong laser configuration for high-speed SS-OCT systems doubles the effective A-line frequency by interleaving orthogonally polarized swept waves within the same cavity (see Goldberg et al “200 kHz A-line rate swept-source optical coherence tomography with a novel laser configuration” Proceedings of SPIE v.7889 paper 55 2011).

[0010] Additionally, Potsaid et al. have demonstrated a technique to double the effective repetition rate of a swept-source laser by buffering and multiplexing the swept waves of a single laser source (see Potsaid et al “Ultrahigh speed 1050 nm swept source / Fourier domain OCT retinal and anterior segment imaging at 100,000 to 400,000 axial scans per second” Opt. Express 18: 20029-20048 2010). However, long fiber spools introduce significant birefringence into the laser output.

[0011] Additionally, there are other architectures for SS-OCT that reduce the performance requirements of the swept laser source. Fechtig et al., in their paper Line-Field parallel swept source MHz OCT for structural and functional retinal imaging, Biomedical Optics Express 716, vol. 6, no. 3, (2015) (Non-Patent Document 9), describe a system that combines a slow sweep speed laser with a linear sensor to achieve an A-scan frequency equivalent to 1 MHz. Examples such as Lee et al.'s Line-Field Optical Coherence Tomography Using Frequency-Sweeping Source, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 14, No. 1, January 2008 (Non-Patent Document 10) are older examples. [Prior art documents] [Non-patent literature]

[0012] [Non-Patent Document 1] Yun et al “High-speed optical frequency-domain imaging” Opt. Express 11:2953 2003 [Non-Patent Document 2] Huber et al “Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines / s,” Opt. Express 13, 3513 2005 [Non-Patent Document 3] Kuznetsov et al “Compact Ultrafast Reflective Fabry-Perot Tunable Lasers For OCT Imaging Applications,” Proc. SPIE 7554:75541F 2010 [Non-Patent Document 4] Bilenca et al “Numerical study of wavelength-swept semiconductor ring lasers: the role of refractive-index nonlinearities in semiconductor optical amplifiers and implications for biomedical imaging applications,” Opt. Lett. 31: 760-762 2006 [Non-Patent Document 5] Kuznetsov et al “Compact Ultrafast Reflective Fabry-Perot Tunable Lasers for OCT Imaging Applications,” Proc. SPIE 7554: 75541F 2010 [Non-Patent Document 6] R. Huber et al “Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines / s,” Opt. Express 13: 3513 2005 [Non-Patent Document 7] Goldberg et al “200 kHz A-line rate swept-source optical coherence tomography with a novel laser configuration” Proceedings of SPIE v.7889 paper 55 2011 [Non-Patent Document 8] Potsaid et al “Ultrahigh speed 1050 nm swept source / Fourier domain OCT retinal and anterior segment imaging at 100,000 to 400,000 axial scans per second” Opt. Express 18: 20029-20048 2010 [Non-Patent Document 9] Fechtig, et al. “Line-Field parallel swept source MHz OCT for structural and functional retinal imaging”, Biomedical Optics Express 716, vol. 6, no. 3, (2015) [Non-Patent Document 10] Lee, et al. “Line-Field Optical Coherence Tomography Using Frequency-Sweeping Source”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 14, No. 1, January 2008 Summary of the Invention [Problem to be solved by the invention]

[0013] The present invention relates to a tunable or swept laser architecture suitable for SS-OCT as well as other applications such as spectroscopy. [Means for solving the problem]

[0014] The swept laser uses a cat's eye configuration with a tilt-tuned filter, preferably a transmissive tilt-tuned filter.

[0015] In one aspect, the invention generally features a tunable laser including a gain chip for amplifying light in a laser cavity, a collimating lens for collimating light from the gain chip, an end reflector for the laser cavity, and a focusing lens for focusing the collimated light onto the end reflector, wherein the wavelength is tunable in combination with a thin film bandpass filter between the collimating lens and the focusing lens, and at least one angle control actuator for changing the angle of the thin film filter relative to the collimated light.

[0016] In one example, the gain chip is a GaAlAs chip that may be packaged in a TO-can type hermetic package and may be a single angled facet edge emitting chip with an anti-reflection coating on the front facet and a curved ridge waveguide that is perpendicular to the back facet but curves at the interface with the front facet.

[0017] The passband of the thin-film bandpass filter may have a full width at half maximum (FWHM) of 0.05 nanometers (nm) to 5 nm wide, or in some cases, a FWHM of 0.1 nm to 2 nm wide.

[0018] In one application, the at least one angle control actuator is a galvanometer. Preferably, it is a servo galvanometer. However, other servo mechanisms and / or motors may be used, such as a motor that continuously rotates the thin film bandpass filter.

[0019] In operation, the at least one angle control actuator tunes the thin film bandpass filter at a tuning speed of 3,000 nm / sec to 11,000 nm / sec, and the tuning range of the thin film bandpass filter is typically 70 nm or greater.

[0020] The size of the beam inside the laser cavity affects performance. Preferably, its diameter is greater than 1 millimeter (mm), and may be greater than 2 mm. The half-cone angle of the collimated light is typically less than 0.04 x 0.02 degrees.

[0021] Another important consideration is polarization: preferably the thin film bandpass filter is oriented to receive S-polarized light from the gain chip.

[0022] In another aspect, the invention generally features a method of controlling a tunable laser including amplifying light in a laser cavity with a gain chip, producing collimated light from the gain chip, focusing the collimated light onto an end reflector, and controlling an angle of a thin film bandpass filter on the collimated light with a servo mechanism.

[0023] The above and further features of the present invention, including various novel structural details, combinations of elements, and other advantages, will now be described with particular reference to the accompanying drawings and the appended claims. It will be understood that the detailed methods and apparatus embodying the invention are merely illustrative and not limiting. The principles and configurations of the present invention may be applied to numerous and varied embodiments without departing from the scope of the invention.

[0024] In the accompanying drawings, reference characters refer to the same structure / component throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. [Brief description of the drawings]

[0025] [Figure 1A] 1 is a schematic side view of a cat's eye type tunable laser according to the present invention; [Figure 1B] FIG. 1 is a schematic side view of a cat's eye type tunable laser using a servo galvanometer. [Figure 2A]FIG. 1 is a schematic plan view of a gain chip for a tunable laser. [Figure 2B] FIG. 2 is a schematic plan view of another gain chip for a tunable laser. [Figure 2C] FIG. 13 is a schematic plan view of yet another gain chip for a tunable laser. [Diagram 3] FIG. 11 is a schematic side view of a cat's-eye type tunable laser according to a second embodiment. [Figure 4] FIG. 11 is a schematic plan view of a cat's-eye type tunable laser according to a third embodiment. [Figure 5A] FIG. 13 is a schematic side view of a cat's-eye type tunable laser according to a fourth embodiment. [Figure 5B] FIG. 11 is another schematic side view of the cat's-eye type tunable laser according to the fourth embodiment. [Figure 5C] 13 is yet another schematic side view of the cat's-eye type tunable laser according to the fourth embodiment. FIG. [Figure 6A] FIG. 2 is a schematic plot of transmission versus frequency illustrating the passband of a bandpass filter. [Figure 6B] FIG. 2 is a plot of tilt angle versus center wavelength of a bandpass filter. [Figure 7] FIG. 1 is a schematic diagram of a swept-source OCT system in which the tunable laser can be used. [Figure 8] FIG. 3 is a diagram of a tunable laser spectrometer 300 using a cat's eye swept laser 100. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. However, the present invention may be embodied in many different forms and should not be construed as being limited to only the embodiments set forth herein. Rather, these embodiments are intended to be thorough and complete, and to fully convey the scope of the invention to those skilled in the art.

[0027] The terms "and / or" and "and / or" as used herein are intended to include any and all combinations of one or more items connected in sequence. Any conjunctions used should be interpreted in the most inclusive sense. That is, the term "or" should be interpreted as having the logical meaning of "logical or" rather than "exclusive or" unless the context clearly indicates otherwise. Furthermore, the singular forms and articles "a", "an" and "the" are intended to include the plural unless otherwise indicated. Additionally, the terms "having" and / or "including" and / or "comprising" as used herein should be understood to specify the presence of the object feature and / or element and / or process and / or treatment and / or component and / or component, and do not exclude the presence or addition of one or more other features and / or elements and / or processes and / or treatments and / or components and / or components and / or combinations thereof. Furthermore, when a component (e.g., part or component of a system) is described and / or illustrated as being connected or coupled to another component, it should be understood that it may be directly connected or coupled to the other component or there may be components therebetween.

[0028] In this specification, even if the terms "first" and "second" are used to describe different components, it should be understood that these components should not be limited by these terms. These terms are used only to distinguish between components. That is, a component described below can be referred to as a "second" component, and a "second" component can also be referred to as a "first" component, without departing from the teachings of the present invention.

[0029] Unless otherwise defined, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by those skilled in the art to which the present invention belongs. Furthermore, even terms defined in dictionaries in common use should be interpreted as having a meaning consistent with the meaning in the context of the relevant technology, and should not be interpreted in an idealized or overly formal way unless otherwise clearly defined in the present specification.

[0030] FIG. 1A illustrates a tunable laser 100, sometimes referred to as a cat's eye laser, constructed in accordance with the principles of the present invention.

[0031] In one example, the laser is amplified by a GaAlAs gain chip 110. The gain chip 110 amplifies light in the wavelength range of about 800-900 nanometers. Preferably, the center wavelength is around 840 nanometers, which has a water window (650-950 nm) and is therefore useful for applications such as ophthalmic imaging and other diagnostic applications. Another advantage of this wavelength range is that it can be detected by standard cameras with silicon-based imager chips. For example, the output can be detected by silicon imaging devices such as complementary metal oxide semiconductors (CMOS) or charge-coupled devices (CCD).

[0032] However, other material systems can be selected for the gain chip. Common material systems include those based on III-V semiconductor materials, such as binary material systems like GaN, GaAs, InP, GaSb, InAs, and ternary, quaternary, and quinary alloys like InGaN, InAlGaN, InGaP, AlGaAs, InGaAs, GaInNAs, GaInNAsSb, AlInGaAs, InGaAsP, AlGaAsSb, AlGaInAsSb, AlAsSb, InGaSb, InAsSb, and InGaAsSb. These material systems generally support operating wavelengths that include long wavelength ranges, such as 400 nanometers (nm) to 2500 nm, spanning several micrometer wavelengths. To obtain very wide gain and spectral emission bandwidths, semiconductor quantum well, quantum cascade, or quantum dot gain regions are typically used, supporting operation at wavelengths up to 250 μm. The quantum well layers can be intentionally strained or unstrained depending on the actual materials and the desired wavelengths of interest.

[0033] In the preferred embodiment, the gain chip 110 is mounted in a TO-can style hermetic package 112, which protects the chip 110 from the humid environment and dust. In some cases, the TO-can style package includes an integrated or separate thermoelectric cooler 114.

[0034] A free-space beam 116 from the package 112 diverges in both the x and y axes. The beam is collimated by a collimating lens 118. The resulting collimated beam is received by a cat's eye focusing lens 120 and focused onto a cat's eye mirror / output coupler 122, which defines the other end of the laser cavity, which extends between the mirror / output coupler 122 and the back / reflective end face of the gain chip 110.

[0035] Between the collimating lens 118 and the cat's eye focusing lens 120, the collimated light 124 passes through a thin film interference bandpass filter 130, which provides a passband with a full width at half maximum (FWHM) of approximately 0.3 nanometers (nm), more broadly between 0.2 nm and 0.5 nm FWHM, more broadly between 0.1 nm and 2 nm FWHM, and even more broadly between 0.05 nm and 5 nm FWHM.

[0036] The bandpass filter 130 is held by the arm of an angle control actuator 132 that changes the angle of the bandpass filter relative to the collimated light 124. In one example, the angle control actuator is a galvanometer. In another example, the angle control actuator 132 is a servo motor or electric motor that continuously rotates the bandpass filter 130 on the collimated beam 124. This allows the bandpass filter 130 to be tilted relative to the collimated beam 124. This tilts the filter, which changes its passband to scan or sweep the wavelength of the swept laser 100.

[0037] Typically, galvanometer tuning speeds are specified in the range of 0.1Hz to 50kHz. For higher speeds, a 25kHz resonant galvanometer with bidirectional tuning can be used; however, faster and slower speeds are also possible. Tuning speeds are usually given in nm / sec, so for a retinal imaging application where a 100kHz line speed camera is used to achieve 1000 bandwidth sampling points and a 70nm tuning range, a 100Hz tuning speed would be ideal: 70nm / 10ms = 7000nm / sec. Typically, tuning speeds of 3,000nm / sec to 11,000nm / sec or greater are desirable.

[0038] For retinal imaging and industrial imaging with inexpensive CMOS cameras, a central wavelength of 840 nm provides an ideal water window. Typically, the minimum tuning range is 30 nm. Preferably, the tuning range is close to or above 60 nm or 70 nm. This provides good resolution in air, below 8 micrometers. A tuning range of 30 nm to 100 nm is typically desired.

[0039] For many applications, the size of the collimated beam 124 is important. As a rule, the smaller the beam, the larger the divergence and the larger the cone half angle (CHA). The diagonal minimum linewidth of the tunable filter is then smaller. In this embodiment, the collimated beam is preferably equal to or larger than 1 millimeter (mm) FWHM, i.e., greater than 1 millimeter (mm) FWHM, and preferably greater than 2 mm FWHM for retinal OCT applications. However, for many infrared, visible or ultraviolet spectroscopy applications, it may be smaller. The CHA is generally less than 0.04×0.02 degrees, preferably about 0.02×0.01 degrees or less.

[0040] The light from the gain chip is polarized. In a typical architecture, the polarization is horizontal or parallel to the epitaxial layers of the edge-emitting gain chip 110. In a preferred configuration, the filter is oriented to receive S-polarized light in order to maintain the narrow linewidth of the filter as it is tilted. On the other hand, as the tilt angle increases, the P-polarized light broadens dramatically. As the tilt angle increases, S-polarized light experiences higher losses than P-polarized light. Therefore, this embodiment seeks to address this issue by designing the filter such that the loss of S-polarized light is sufficiently low over the entire tuning band.

[0041] On the other hand, for spectroscopy applications, a P-polarized configuration may be desirable for high power across the entire scan band.

[0042] In general, the cat's eye configuration of the present invention has a number of advantages: It has smaller angle-wavelength variations than grating-based lasers, resulting in stable operation with low losses, low tolerances and high repeatability.

[0043] Typically, mirror / output coupler 122 reflects approximately 80% of the light back into the laser cavity and transmits approximately 20% of the light. More generally, the reflection (transmission) of light by the mirror / output coupler can be anywhere from 10% to 99% reflection (90% to 1% transmission) of light depending on the desired output power and laser cavity losses. Higher reflectivity results in lower cavity losses and a wider range of laser tuning over which gain exceeds loss, but at the expense of lower output power. In typical operation, mirror / output coupler 122 is less than 90% reflective.

[0044] In some embodiments, an iris or mask 190 is typically added after the mirror output coupler 122 to clip the beam edges. This reduces power fluctuations as the beam wanders due to refraction in the bandpass filter 130 during tilting. Preferably, the beam size is 80%-95%, preferably about 90%.

[0045] In general, the diverging beam from the mirror output coupler 122 is typically collimated by an output collimating lens 140 to form the free-space output beam 102 .

[0046] FIG. 1B illustrates one preferred application of the tunable laser 100, and in particular, the angle control actuator 132.

[0047] The angle control actuator 132 operates as a servo mechanism. In the illustrated embodiment, the angle control actuator 132 is a servo controlled galvanometer with an encoder 160. The encoder 160 generates an angle signal 162 indicative of the angle of the galvanometer, and thus the filter 130, relative to the collimated beam 124. Preferably, the encoder is an optical encoder, and often is analog.

[0048] The control / processing unit receives the angle signal 162 at a PID (Proportional-Integral-Derivative) controller 164. The PID controller 164 compares the angle signal 164 to a specified regulation function, which may be a sawtooth or triangular waveform. The PID controller 166 generates a control function 168 that is used to drive the windings of the galvanometer 132 across an amplifier 169.

[0049] A preferred gain chip architecture is shown in Figure 2A. The chip 110 is referred to as a single angled facet (SAF) edge emitting chip. That is, its rear facet 150 is highly reflective (HR) coated. Its front facet 152 is anti-reflective (AR) coated. Additionally, to improve performance, it has a curved ridge waveguide 154 that is perpendicular to the rear facet 150 but bends at the interface with the front facet 152. This bend and AR coating on the front facet significantly improves laser performance by reducing reflections due to the reflectivity of the front facet by up to 40 dB and suppressing parasitic reflections that can lead to non-smooth tuning and mode hopping.

[0050] Another possible edge-emitting gain chip configuration is shown in Figure 2B. The basic configuration is what is called a semiconductor optical amplifier (SOA) gain chip: the back facet is AR coated and the front facet is also AR coated. A straight but tilted ridge waveguide 156 intersects each facet at an angle to minimize reflections back into the chip. In one example, the light at the back facet is coupled into a lens or pair of lenses and a mirror that reflects the light back through the lens and into the chip. The mirror may be a partially reflective mirror that extracts the output from the back facet.

[0051] Yet another possible gain chip configuration is shown in Figure 2C. The basic configuration is what is called a Fabry-Perot gain chip: the rear facet 150 is HR coated and the front facet 152 is AR coated. A straight ridge waveguide 158 intersects the front facet 152 at a right angle, which can cause some internal reflections that affect performance.

[0052] Another example of a laser 100 is shown in FIG. 3, in which one or more outputs are extracted from the laser cavity. Specifically, an angled beam splitter 142 extracts a portion of the light in the laser cavity as a first output and a second output, which are collimated output beams. An end mirror 144 is typically highly reflective, such as greater than 99%, unless a third output is provided.

[0053] As mentioned above, the output coupler may be implemented as a beam splitter. The output coupling is selected by making the output coupler have the desired reflectance-to-transmittance ratio. Another option is to use a polarizing beam splitter in combination with a quarter wave plate. The angle of the quarter wave plate can then be controlled to make the output coupling adjustable.

[0054] There are two actual outputs in this configuration: a collimated first output and a collimated second output 2. The collimated first output is generally higher power since it receives light directly from the chip. It also features a strong amplified spontaneous emission (ASE) spectrum. On the other hand, the collimated second output can be higher or lower power, but it receives light after a double pass through the bandpass filter and therefore features weaker spectral sidebands. Note that in this configuration, the position of the output light does not shift even if the angle of the filter is adjusted, since the reflected light travels back through the filter and follows itself.

[0055] In this example, it is possible to incorporate a k-clock, by adding an etalon to one output, and the trigger signal generated can be used by the camera to efficiently sample without the need for software resampling.

[0056] Another embodiment of a swept laser is shown in Figure 4. In this version, the linewidth of a bandpass filter 130 is narrowed by rotating it in the image plane of the figure with a rotary actuator 132. In particular, the light from the TO-CAN 112 is collimated by an internal lens.

[0057] The linewidth is narrowed with a six-pass arrangement. Specifically, the laser cavity is expanded with two retroreflectors 146, 148, allowing six passes through the bandpass filter in one pass of the cavity. In the example shown, an output coupler 142 is used within the cavity. However, in other examples, a cat's eye mirror / output coupler is used. Another four-pass arrangement has a cat's eye reflector on the same side of the bandpass filter as the input beam.

[0058] FIG. 5A shows another embodiment of a cat's eye swept or tunable laser.

[0059] A free space beam 116 from the package 112 is collimated by a collimating lens 118. The beam passes through an optional output coupler beam splitter 142. The collimated beam is then reflected off a first tilt mirror 170. A first galvanometer 172 controls the tilt angle of the first tilt mirror 170 in the drawing plane.

[0060] The light is then received by a first mirror focusing lens 174, which is a focal length f1 away from the first mirror. A filter focusing lens 176 directs the light through to a bandpass filter 130, which in this example is fixed. The filter focusing lens 176 is a focal length f2 away from the bandpass filter 130. A filter focusing lens 178 is a focal length f3 away from the bandpass filter 130 and collimates the light from the bandpass filter. A cat's eye focusing lens 180 then focuses the light onto a cat's eye mirror and an optional output coupler 182, which is held by a second galvanometer 184. The cat's eye focusing lens 180 is a focal length f4 away from the cat's eye tilt mirror 182.

[0061] Fig. 5B shows how this embodiment of the tunable laser is tuned to another wavelength. The first and second galvanometers are driven synchronously so that the light beam retraces its path. Since the light beam passes through the bandpass filter at an angle, tilt tuning is achieved without moving the bandpass filter.

[0062] 5C shows this tunable laser embodiment being tuned to yet another wavelength. Again, the first and second galvanometers are driven synchronously so that the light beam retraces its path. As the light beam passes through the bandpass filter 130 with a further tilt, even more tilt adjustment is achieved without moving the bandpass filter.

[0063] 6A is a plot of transmission versus frequency for collimated light between the collimating lens and a cat's eye focusing lens with a passband filter at a particular angle. It can be seen that the passband is narrow.

[0064] 6B is a plot of passband wavelength for S-polarized light versus beam angle for filter 130. It can be seen that the passband can be adjusted by tilting the passband filter with a galvanometer.

[0065] The following equation relates the passband wavelength to the center wavelength in the untilted state:

[0066]

number

[0067] (where θ is the angle the beam makes with the filter.)

[0068] The above formula shows that the filter tilt angle is slow in the small angle range and then becomes fast. Operation in the linear region is preferred to reduce the required tilt angle and to make the scan more linear. The filter function shown is for a laser operating in the tuning range of 810-870 nm. And for the input wavelength at 0 degrees, a wavelength of 900 nm is chosen. This results in a minimum operating angle around 870 nm, and tuning is performed at 810-870 nm. Note that the angle adjustment is always performed to shorten the wavelength. N eff is adjustable and can serve to amplify the angular adjustment.

[0069] 7 illustrates a swept-source optical coherence tomography system (SS-OCT) 200. In the illustrated example, the OCT system 200 is used for ophthalmic analysis of a human eye 202, and in particular, a retina 204.

[0070] The OCT system 200 comprises a cat's eye swept laser 100, preferably equipped with a servo galvanometer to provide highly repeatable swept source operation. Light from the laser 100 in the form of a free space beam 102 passes in free space to line forming optics 208 and then to a beam splitter 210, such as a cube beam splitter of an OCT interferometer.

[0071] Typically, the line forming optics 208 comprises one or more cylindrical lenses and / or other lenses in a beam expander configuration. The line forming optics 208 converts the light from the laser 100 into a line (more specifically, a rectangular profile) with an aspect ratio of at least 10:1, typically greater than 100:1, and often greater than 400:1. That is, the light from the line forming optics 208 appears as a line (more specifically, a rectangular two-dimensional profile) with one dimension at least 10 times larger than the other dimension when viewed along its optical axis.

[0072] A beam splitter 210 splits the light into a reference arm 212 and a measurement arm 214 in a Michelson arrangement as shown. The light propagates in free space between one or more lenses forming a projection / collection optics 222 in the measurement arm, illuminating a measurement object 202. A typical measurement object is human eye tissue, such as the retina 204.

[0073] Typically, the light is scanned across the measurement object by a galvanometer-driven scanning mirror 220 between the beam splitter 210 and the measurement object 202. The scanning mirror performs the scan such that the light beam moves perpendicular to the long axis of the rectangular beam profile.

[0074] In the reference arm, light conditioned by one or more lenses in the reference arm optics 224 is reflected off of a reference mirror 226 .

[0075] The measurement light collected and received via the light projecting and collecting system 222 is combined with the light from the reference arm to produce optical interference in the line scan sensor 230. Typically, the line scan sensor has a linear array of at least 512 pixels, and often at least 1024 or 2048 pixels.

[0076] It is important to note in the illustrated example that the light from the cat's eye swept laser 100 travels in free space through the cube beam splitter 210 and the lenses of the line forming optics 208, collection optics 222 and reference arm optics 224, via the OCT interferometer to the line scan sensor 230. No waveguides such as optical fibers are present.

[0077] The output from the sensor 230 is read out by a processor 232. The results can be stored therein and / or displayed on a display 234. A Fourier transform of the coherent light reveals a profile of the scattering intensity at each path length, i.e., scattering as a function of depth (z-direction) within the object (see, e.g., Leitgeb et al, “Ultrahigh resolution Fourier domain optical coherence tomography,” Optics Express 12(10):2156 2004). The scattering profile as a function of depth is referred to as an axial scan (A-scan). A series of A-scans measured at successive positions within the object produces a cross-sectional image (tomogram or B-scan) of the object. A collection of B-scans constructs a data cube or cube-scan.

[0078] 8 shows a tunable laser spectrometer 300 using a cat's eye swept laser 100. The chip 110 is typically an InP or GaSb SAF chip.

[0079] In this example, a free-space beam 102 from a cat's eye swept laser 100 illuminates a sample, such as a gas or liquid, in a sample cell 310. Light from the sample cell is detected by a detector 312.

[0080] The processor 232 controls the sweep of the tunable laser, in particular its servo galvanometer, over the tuning range of the laser. Preferably, the tuning range is 20 nm or more. Typically, tuning ranges of more than 60 nm or more than 70 nm are suitable. In general, a tuning range of 50 nm to 300 nm is desirable. In addition, the processor 232 resolves the absorption spectrum of the sample in the sample cell 310 by monitoring the time response of the detector 312.

[0081] While the present invention has been particularly shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as encompassed by the appended claims.

Claims

1. a gain chip for amplifying light within the laser cavity; a collimating lens for collimating light from the gain chip; an end reflector of the laser cavity; a condenser lens that condenses the collimated light onto the end reflector; a thin-film bandpass filter between the collimating lens and the condensing lens; at least one angle control actuator for varying the angle of the thin film filter relative to the collimated light; 1. A tunable laser comprising:

2. 2. The tunable laser of claim 1, wherein said gain chip is a GaAlAs chip.

3. 3. The tunable laser according to claim 1, wherein the gain chip is mounted in a TO-CAN type hermetic package.

4. 4. The tunable laser of claim 1, wherein the passband of the thin film bandpass filter is 0.05 nanometers (nm) to 5 nm wide full width at half maximum (FWHM).

5. 5. The tunable laser according to claim 1, wherein the passband of the thin film bandpass filter is 0.1 nm to 2 nm wide FWHM.

6. 6. The tunable laser of claim 1, wherein the at least one angular control actuator is a galvanometer.

7. 7. The tunable laser of claim 1, wherein the at least one angle control actuator is a servomechanism.

8. 6. The tunable laser according to claim 1, wherein the at least one angle control actuator is a motor that continuously rotates the thin film bandpass filter.

9. 9. The tunable laser according to any one of claims 1 to 8, wherein the at least one angle control actuator tunes the thin film bandpass filter with a tuning speed of 3,000 nm / sec to 11,000 nm / sec.

10. 10. The tunable laser according to claim 1, wherein the at least one angle control actuator has a tuning range of 70 nm or more by adjusting the thin film bandpass filter.

11. 11. The tunable laser of any one of claims 1 to 10, wherein the diameter of the collimated light is greater than 1 millimeter (mm).

12. 11. The tunable laser according to claim 1, wherein the diameter of the collimated light is greater than 2 mm.

13. 13. The tunable laser of any one of claims 1 to 12, wherein the cone half angle of the collimated light is less than 0.04 x 0.02 degrees.

14. 14. The tunable laser of any one of claims 1 to 13, wherein the thin film bandpass filter is oriented to receive S-polarized light from the gain chip.

15. 15. A tunable laser according to any one of claims 1 to 14, wherein the gain chip is an edge-emitting chip with an anti-reflective coating on its front facet and a single angled facet having a curved ridge waveguide perpendicular to the rear facet but curved at the interface with the front facet.

16. 16. The tunable laser according to claim 1, wherein the angle control actuator tilts the thin film bandpass filter between the collimating lens and the focusing lens to achieve a tuning speed of 3,000 nm / sec to 11,000 nm / sec.

17. amplifying light within the laser cavity with a gain chip; generating collimated light from the gain chip; focusing the collimated light onto an end reflector; controlling an angle of a thin film bandpass filter on the collimated light by a servo mechanism; A method for controlling a wavelength tunable laser comprising:

18. 18. A method according to claim 17, using a laser according to any one of claims 1 to 16.

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