Abstract:
The present invention provides a swept fiber optic laser source for optical coherence tomography emitting around ˜1060 nm wavelength, with tuning range higher than 50 nm, sweep repetition rate from DC to 40 kHz, instantaneous linewidth shorter than 50 pm (FWHM), and providing an average output around 1 mW (or 20 mW with output optical booster amplifier). The fiber laser source is based on a proper linear-cavity fiber laser configuration, with an intra-cavity half-symmetrical confocal Fabry-Perot tunable fiber (FP-TFF) filter and semiconductor optical amplifier (SOA), a device combination that gives a very robust and vibration-resistant laser configuration.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    This invention relates to optical coherence tomography and more specifically, to a swept fiber optic laser source centered near a wavelength of 1060 nm with a tuning range higher than 50 nm, a sweep repetition rate from DC to 40 kHz, an instantaneous line-width shorter than 40 pm (FWHM), and providing an average output power around 1 mW (or 20 mW with an output optical booster amplifier). 
         [0003]    2. Description of Related Art 
         [0004]    Commercially available retinal ophthalmic optical coherence tomography (OCT) systems operate at a central wavelength of approximately 820 nm due to the relatively low cumulated absorption of the eye tissue at this wavelength. Although the majority of retina imaging reports refer to this band, and ultrahigh resolution has also been demonstrated in this wavelength region for resolving intra-retinal layers, it has limited depth penetration beyond retinal pigment epithelium (RPE). For imaging features beyond the RPE, longer wavelengths are more suitable. See Unterhuber et al., “In vivo retinal optical coherence tomography at 1040 nm-enhanced penetration into the choroids,” Optics Express 13(9), pp. 3252-3258 (2005), the disclosure of which is incorporated by reference herein in its entirety. This relates to the fact that the absorption and scattering properties of melanin (the main chromophore in the RPE) tend to decrease with increasing wavelength. 
         [0005]    Water absorption, on the other hand, represents a more critical limitation especially when imaging a biological sample because of its high content (˜90%) of water. There is, however, a spectral window restricted to a wavelength span of 100 nm (a band from 1 μm-1.1 μm) where the water absorption spectrum exhibits a minimum value. Moreover, the optical power loss due to increased water absorption compared to the 800 nm band is compensated by the fact that the corneal maximum permissible exposure for longer wavelengths also increases according to American National Standards Institute (ANSI) and International Electrotechnical Commission (IEC) standards. An additional advantage of optical imaging at 1060 nm wavelength band is the zero dispersion point of water, which eliminates the depth dependent broadening of axial resolution over reasonable depth penetration. See Pova{hacek over (z)}ay et al., “Enhanced visualization of choroidal vessels using ultrahigh resolution ophthalmic OCT at 1050 nm,” Optics Express 11, pp. 1980-1986 (2003), the disclosure of which is incorporated by reference herein in its entirety. 
         [0006]    Commercial OCT systems traditionally use 850 nm broadband light sources providing a wide range of emission wavelengths, all simultaneously. The OCT systems that use 1050 nm broadband light sources need an expensive InGaAs linear camera for photodetection, which greatly increases the overall cost of the unit. With a wavelength swept source, interference signals at individual wavelengths can be measured sequentially with high spectral resolution and with an inexpensive InGaAs photodetector. This method offers significantly higher sensitivity than the traditional time-domain used in conventional OCT systems with broadband sources. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention overcomes these and other deficiencies of the prior art by providing a swept fiber optic laser source emitting around ˜1060 nm wavelength, with tuning range higher than 50 nm, sweep repetition rate from DC to 40 kHz, instantaneous linewidth shorter than 50 pm (FWHM), and providing an average output around 1 mW (or 20 mW with output optical booster amplifier). The fiber laser source is based on a proper linear-cavity fiber laser configuration, with an intra-cavity half-symmetrical confocal Fabry-Perot tunable fiber (FP-TFF) filter and semiconductor optical amplifier (SOA)—a device combination that gives a very robust and vibration-resistant laser configuration. 
         [0008]    In an embodiment of the invention, a swept fiber laser source comprises: an optical amplifier, a mirror optically coupled to a fiber end of the optical amplifier, an optical circulator optically coupled to another fiber end of the optical amplifier, a tunable fiber filter optically coupled to the optical circulator, and a fiber coupler comprising a first input port and a second input port, the first input port being optically coupled to the optical circulator and the second input port being optically coupled to the tunable fiber filter. The optical amplifier comprises a semiconductor optical amplifier. The swept fiber laser source may further comprise a fiber pigtailed polarizer element located between the tunable fiber filter and the fiber coupler. The tunable fiber filter may comprise an intra-cavity half-symmetrical confocal Fabry-Perot tunable fiber (FP-TFF) filter or a multi-inference filter selected from the group consisting of: a fiber Bragg grating, a flat fiber Fabry-Perot filter, an acousto-optic tunable filter, a microelectromechanical system (MEMS) tilt filter, and a combination thereof. The source has an adjustable output wavelength between 1010 nm and 1110 nm, a linewidth less than 50 pm at full width half maximum, and an output power of 1 mW. The swept fiber laser source may further comprise an optical amplifier booster stage optically coupled to an output port of the fiber coupler to boost the output power to 20 mW. The optical amplifier may also comprise an optical fiber gain media section including a single mode Yb-doped fiber optically coupled at one end to the mirror, a second fiber coupler optically having a first input port, second input port, and an output port, the first input port being optically coupled to the single mode Yb-doped fiber and the output port being optically coupled to the optical circulator, and a pump source optically coupled to the second input port of the second fiber coupler. The pump source comprising a laser diode at 975 nm The coupler has a 30/70 coupling ratio between the first input port and the second input port, respectively. The optical circulator may comprises a polarization maintaining isolator, which includes a blocking arm to eliminate a polarization. The swept fiber laser source may be implemented in an optical coherence imaging system. 
         [0009]    In another embodiment of the invention, a swept fiber laser source comprises: an optical fiber gain media, a mirror optically coupled to the optical fiber gain media, an optical circulator optically coupled to the optical fiber gain media, a tunable fiber filter optically coupled to the optical circulator, and a fiber coupler comprising a first input port and a second input port, the first input port being optically coupled to the optical circulator and the second input port being optically coupled to the tunable fiber filter. The optical fiber gain media comprises: a single mode Yb-doped fiber, a pump coupler, and a pump source. The single mode Yb-doped fiber has a peak absorption coefficient of 250 db/m at 975 nm and a length greater than 8 meters. The pump source comprises a pump laser diode emitting light at 975 nm. 
         [0010]    Compared with laser ring cavity lasers, linear cavity lasers have the advantage that the gain medium amplifies the laser light twice per circulation, thus making it easy to reach deep saturation. Therefore, large tuning ranges as well as a low threshold pump power and high slope efficiency can be easily achieved. 
         [0011]    The foregoing, and other features and advantages of the invention, will be apparent from the following, more particular description of the preferred embodiments of the invention, the accompanying drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    For a more complete understanding of the present invention, the objects and advantages thereof, reference is now made to the ensuing descriptions taken in connection with the accompanying drawings briefly described as follows: 
           [0013]      FIG. 1  illustrates a swept fiber laser source according to an embodiment of the invention; 
           [0014]      FIG. 2  illustrates a swept fiber laser source comprising a booster stage according to an embodiment of the invention; 
           [0015]      FIG. 3  illustrates a plot of continuous wave (CW) emission spectra of the swept fiber laser source of  FIG. 1 ; 
           [0016]      FIG. 4  shows three plots of the sweeping optical spectra as a function of sweeping rate according to an embodiment of the invention; 
           [0017]      FIG. 5  shows a plot comparison of the continuous wave (CW) emission spectra of the swept fiber laser source with and without an optical amplifier booster; 
           [0018]      FIG. 6  shows the wavelength tuning range curve of the swept fiber laser according to an embodiment of the invention; and 
           [0019]      FIG. 7  illustrates a swept fiber laser source according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0020]    Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying  FIGS. 1-7 , wherein like reference numerals refer to like elements. Although the fiber source is described in the context of optical coherence tomography, one of ordinary skill in the art readily appreciates that the present invention can be implemented in any type of system where it is desired to implement a swept fiber laser source with a wavelength emission centered near 1060 nm, for example, optical sensing (strain and temperature). 
         [0021]      FIG. 1  illustrates a swept fiber laser source  100  according to an embodiment of the invention. The swept fiber laser source  100  comprises a semiconductor optical amplifier (SOA) device  150  spliced at one fiber end to a common-port of a polarization-maintaining (PM) optical circulator  140 . The other fiber end of the SOA device  150  is spliced to a PM fiber mirror  160 . The input port of the PM optical circulator  140  is connected to a PM single mode fiber  110 , which is also connected to one port of an output of a PM fiber coupler  130  with a tailored coupling ratio, such as a 30/70 ratio (other coupling ratio values such as, but not limited to 50/50, 20/80, 10/90 can also be used in order to adjust the overall optical power of the swept fiber laser source  100 ). The output port of this output PM fiber coupler  130  is spliced to a PM fiber connector  170 , such as a ferrule connector with an angled physical contact (FC/APC) or fiber connector angle (FCA). The second input port of the output PM fiber coupler  130 , is connected to the output-port of a PM tunable fiber filter  120 , which may comprise an intra-cavity half-symmetrical confocal Fabry-Perot tunable fiber (FP-TFF) filter. However, one of ordinary skill in the art readily appreciates that other tunable multi-inference fiber filters, such as, but not limited to a fiber Bragg grating, a flat fiber Fabry-Perot filter, an acousto-optic tunable filter, a microelectromechanical system (MEMS) tilt filter, or a combination thereof can be implemented in place of the FP-TFF filter. 
         [0022]    Fibers in the present invention preferably comprise polarization-maintaining (PM) single mode silica fiber such as Nufern PM 980 fiber. In another embodiment of the invention, non-PM single mode fibers may be implemented, but the stability of the optical output of the fiber laser  100  may drift, imposing the need to control the polarization state of the light through the laser cavity. Specifically, the round-trip cavity eigenmodes will split into two orthogonal elliptically polarized modes. These modes will settle in polarization directions and states depending on local stress axes in the fiber or in any of the fiber pigtailed elements. Hence, the polarization axis of the circulating light is expected to stay stable, but may wonder depending on various factors such as thermal or internal stress factors. Their axis of orientation would appear to be arbitrary, but will depend on the orientation of local stress factors in the mirrors, SOA or couplers. 
         [0023]    In an embodiment of the invention, the SOA device  150  features a small signal gain peak of 27.5 dB at 1060 nm. Small signal refers to the condition where the input power is low, population inversion is high, and gain is still at max or close to max, such as when the input power is a few microwatts. Two advantages of using a SOA device  150  as a gain media are: (1) high gain (typically 20 to 35 dB) with a broad spectral bandwidth, and (2) the gain response time of a SOA is about 250 ps, much shorter than the micro- or millisecond timescale of other gain media such as Ti:Sapphire and rare-earth doped fibers. For a fast sweep operation, the short gain response time is highly desirable to minimize the intensity noise of the laser output. 
         [0024]      FIG. 2  illustrates a swept fiber laser source  200  including a booster stage according to an embodiment of the invention. Particularly, swept fiber laser source  200  comprises an optical amplifier booster stage  280  disposed between before the PM fiber connector  170 , such as a ferrule connector with angled physical contact (FC/APC) or fiber connector angle (FCA). In an exemplary embodiment of the invention, the optical amplifier booster stage  280  comprises an Yb-doped fiber amplifier booster. In another embodiment of the invention, the optical amplifier booster stage  280  comprises a semiconductor optical amplifier (SOA) booster, which is relatively smaller and more compact than a fiber amplifier. A booster amplifier operates in the saturated-gain regime and has a gain range from 3 to 10 dB, which is lower than those operating in small signal. 
         [0025]      FIG. 3  illustrates a plot of continuous wave (CW) emission spectra  300  of the swept fiber laser source of the present invention without a booster amplifier. This emission spectra  300  is centered around 1055 nm and has a full width at half maximum (FWHM) less than 50 pm, emitting an average optical power around 1 mW and having a side-mode suppression ratio (SMSR) better than 55 dB. The amplified spontaneous emission (ASE) noise is very small because the PM tunable fiber filter  120  is placed before the output PM fiber coupler  130  as shown in  FIG. 1 . The CW emitted center wavelength of this swept fiber laser source is tunable (with a range higher than 50 nm) and is controlled by the amplitude of the DC voltage applied to the PM tunable fiber filter  120 . 
         [0026]      FIG. 4  shows three normalized plots  400 ,  410 , and  420  of the sweeping optical spectra of the present invention without a booster amplifier obtained from an optical spectrum analyzer. Specifically, the spectrum analyzer operated in peak-hold mode with 1 nm resolution. The plot  400  represents a sweep repetition rate of 1 kHz for the fiber laser. The plots  410  and  420  represent a sweep repetition rate of 10 kHz and 20 kHz, respectively. The present swept fiber laser source is capable of sweeping repetition rates up to 40 kHz, but with some optical power reduction as seen in  FIG. 4 , between the traces  400 ,  410  and  420 . Here, one of ordinary skill in the art can appreciate the relatively large tuning range, which is attractive for many applications including OCT. 
         [0027]      FIG. 5  shows a plot comparison of the continuous wave (CW) emission spectra of the swept fiber laser source with and without optical amplifier booster  280 . Specifically, plot  500  represents the swept fiber laser source  200  with the presence of the optical amplifier booster  280 . The plot  510  represents the swept fiber laser source  100  (without the presence of the booster  280 . Due to the amplified spontaneous emission (ASE) noise of the optical amplifier booster  280 , the SMSR of the amplified output signal  500  will be reduced (being lower than 40 dB), but the output power is higher, reaching values around 20 mW and still maintaining the FWHM linewidth of the swept fiber laser of the present invention. As shown, the plot  510  was generated at 1065 nm This shift is not critical and simply indicates that a different voltage was applied at the time to the tuning device, or that the tuning device has drifted in wavelength. 
         [0028]      FIG. 6  shows the wavelength tuning range curve  600  of the swept fiber laser according to an embodiment of the invention. The tuning range curve  600  is a function of the amplitude of the swept signal applied (sinusoidal signal at 1 kHz) to the tunable fiber filter. This results show that the swept fiber laser has good bidirectional linearity with a tuning coefficient of about 5.2 nm/V. 
         [0029]      FIG. 7  illustrates a swept fiber laser source  700  according to an embodiment of the invention. The swept fiber laser source  700  comprises an optical fiber gain media section  710  instead of the SOA gain media  150 , which was implemented in the fiber laser source  100 . The gain media section  710  comprises a PM single mode Yb-doped fiber  770  as a gain media combined with a PM pump fiber coupler  750  and pump source  760 . The PM single mode Yd-doped fiber  770  has a peak absorption coefficient of 250 dB/m at 975 nm band and measures 8-10 m in length. One end of this PM single mode Yb-doped fiber is spliced to a PM pump fiber coupler  750 , such as a 975/1060 thin-film or fused fiber coupler, and the other end is spliced to the PM fiber minor  160 . The pump source  760  comprises a pump laser diode at 975 nm single mode or multimode, and emits a pump signal that is coupled in the input fiber port of the 975/1060 pump coupler  750 , and propagates into the PM single mode Yb-doped fiber  770 , where it is absorbed by and excites the ytterbium dopant ions in the fiber core to reach an electronic population inversion that will able to amplify the optical energy. 
         [0030]    The configuration of the swept fiber laser source  100  is ideal and designed to produce very good performance. However, one can produce a similarly tunable laser, but with slightly diminished extinction ratio and more noise if one were to introduce some simplifications to the components. For example, according to an embodiment of the invention, the components of the fiber laser source  100  are non-PM. This simplifies and reduces the cost of many of the components. In another embodiment of the invention, the circulator  140  comprises one or more polarization maintaining isolators. To introduce polarization selection and blocking, one of these isolators, preferably the one associated with the fiber arm that is connected to the filter element  120 , contains a blocking arm to eliminate one of the two polarizations. This polarization loss factor, would force the system to operate at the unattenuated polarization, and with almost the same power and SNR. In another embodiment of the invention, the circulator  140  is a non-PM circulator and a fiber pigtailed polarizer element is introduced anywhere in the  FIG. 1 , but preferably between the PM tunable fiber filter  120  and the output PM fiber coupler  130 . The preferred polarization axis for this element would be adjusted (aligned) to align the output polarization to the desired axis at the PM fiber connector  170 . This location would produce the best SNR and polarization extinction signal at the PM fiber connector  170 . In another embodiment of the invention, no polarization selection or discrimination is introduced. The output signal at the fiber connector  170  therefore does not have a well defined polarization axis. Nevertheless, the output signal is polarized in some random polarization state. Its wavelength however is still widely tunable and selectable by the tunable fiber filter  120 . 
         [0031]    The invention has been described herein using specific embodiments for the purposes of illustration only. It will be readily apparent to one of ordinary skill in the art, however, that the principles of the invention can be embodied in other ways. Therefore, the invention should not be regarded as being limited in scope to the specific embodiments disclosed herein, but instead as being fully commensurate in scope with the following claims.