Wavelength scanning light source

Abstract

A wavelength-scanning light source is based on a combination of an ultrashort optical pulse coherent source, a doped fiber amplifier, and a dedicated dispersive optical medium to generate time-stretched pulses. The pulses are broadened to have a spectral bandwidth covering a wavelength range of interest for a specific wavelength-scanning application and are thereafter subjected to time stretching within the dispersive optical medium so as to sufficiently separate in time the multiple wavelength components within each pulse.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 754,082, filed on Nov. 1, 2018, which is hereby incorporated herein by reference. Technical Field

[0003] The present invention relates to a wavelength - scanning light source, and more particularly to a light source capable of providing wavelength - scanned output over a wide spectral range at a relatively fast scan rate, which scan rate is useful for imaging, sensing, and spectroscopy applications (e.g., scan rate exceeding 2 MHz). Background Art

[0004] In addition to using optical systems for communication applications, the use of laser - based devices in imaging, sensing, and spectroscopy applications has proven to be a valuable technique for capturing and analyzing data. In various such systems, a light source having what is sometimes referred to as a “broadband” light source and more appropriately characterized as a “wavelength - scanning” light source is useful, where a series of light beams at a defined set of wavelengths are used to illuminate a given object. Since the response of the object is typically a function of the wavelength of the illuminating light, the act of “scanning” a set of different wavelengths across the object provides a wavelength - dependent response that can, for example, sense the presence of a toxic gas, identify the presence of a minor deformation in a bridge span, or even help characterize a tumor found in the human body.

[0005] To provide consistent and repeatable results, it is important that the laser source used to generate the scanned - wavelength output exhibit as high a coherence level as possible. In fact, some applications may require a coherence length of at least 1 mm (“coherence length” is the span over which there is a well - defined phase relationship between the start and end of a propagating light wave). A common choice for a laser source that provides a scanned - wavelength output with such a coherence level is a “Fourier - domain mode - locked” (FDML) laser. In an FMDL laser, the output wavelength is changed by tuning a variable band - pass filter placed within the laser cavity. Tuning typically involves some type of actuation to adjust the center wavelength of the filter (usually mechanical, or sometimes thermal), thus not only limiting the scan speed (i.e., the time required to scan across the wavelength range from one end to the other), but also limiting the “duty cycle” of the source, since the tunable filter needs to be reset to its initial wavelength value before starting the next scan.

[0006] Thus, the need for any type of external actuator / filter mechanism to control the wavelength - scanning light source inherently limits the scan rate and / or bandwidth that can be achieved, especially since most attempts to improve its performance increase the complexity, size, and cost of the final product. Summary of the Invention

[0007] The present invention relates to a wavelength-scanning light source, and more particularly to a fiber-based light source that is capable of providing wavelength-scanned output over a wide spectral range at a scan rate far exceeding that of prior art devices without the need to perform any actuation-based tuning of the output wavelength.

[0008] In accordance with the principles of the present invention, the wavelength-scanning light source is formed by a combination of a coherent pulsed laser source, a fiber-based optical amplifier, and a dispersive optical medium (in most cases, implemented as a section of dispersive fiber). The parameters of these elements are coordinated such that the output from the dispersive optical medium consists of a series of "time-stretched" pulses, where the selected wavelength components within a given stretched pulse exit the light source at measurably different (i.e., "distinct") time points. By mapping a set of wavelength components to specific arrival times via dispersive Fourier transform (DFT) techniques, an instrument placed at the output of the wavelength-scanning light source will be able to correlate the time series with the defined wavelength components within each time-stretched pulse exiting the wavelength-scanning light source.

[0009] Advantageously, the use of "time-stretched" pulses to generate the wavelength-scanned optical output eliminates the need to use a tunable bandpass filter to generate the wavelength scan, thereby allowing a significant increase in the scan rate relative to prior art configurations. Moreover, since there is no need to manually "reset" the tunable filter between cycles, the wavelength-scanning light source of the present invention is capable of utilizing an input pulse source with a higher repetition rate than prior art. In fact, a repetition rate of 4.7 MHz was used in the testing of an exemplary fiber-based wavelength-scanning light source of the present invention.

[0010] The operating parameters of the various elements of the wavelength-scanning light source of the present invention are coordinated to provide an acceptable level of output power uniformity across the bandwidth range of interest. For example, embodiments of the present invention are capable of achieving less than 10 dB of power spectral density (PSD) variation over a relatively wide spectral range by appropriately selecting the operating parameters of the doped fiber amplifier element.

[0011] In one or more embodiments, the coherent pulsed laser source may include a mode-locked fiber laser (e.g., a figure-eight fiber laser) to provide ultrashort (less than 1 ps) "seed" pulses as the input to the amplifier element.

[0012] The dispersive optical medium may include a fiber, waveguide, bulk optical device, or any other medium suitable for supporting the propagation of optical signals. In a preferred embodiment, the dispersive optical medium is preferably configured to exhibit a total dispersion that provides a duty cycle close to one. For the purposes of the present invention, the term "duty cycle" as used herein is defined as the ratio of the time (t sweep ) required to perform a complete wavelength scan to the complete cycle time interval (t cycle ).

[0013] Exemplary embodiments of the present invention may take the form of a wavelength-scanning light source that includes a laser source of optical pulses (preferably ultrashort pulses), a doped fiber optical amplifier, and a dispersive optical medium located at the output of the doped fiber optical amplifier. The doped fiber amplifier generates a spectrally broadened output pulse in response to both the optical pulse and a pump beam (of a selected wavelength and power), and the output pulse has a minimum variation in power spectral density over a predetermined bandwidth within the spectral broadening bandwidth. The dispersive optical medium is configured to have an average dispersion D per unit length avg and a predetermined length L DF (defined as D avg *L DF for the total dispersion D tot ), sufficient to "time stretch" the amplified pulses from the doped fiber optical amplifier such that different wavelength components within the pulse exit the dispersive optical medium at different time points.

[0014] Another embodiment of the present invention relates to a method of generating a wavelength-scanning optical output from a light source, the method comprising the steps of providing a series of optical pulses at a predetermined repetition rate, applying the optical pulses as an input to a fiber-based optical amplifier, amplifying the optical pulses and broadening each pulse to span a predetermined spectral bandwidth, and then passing each pulse through a dispersive optical medium having a predetermined average dispersion D avg and a predetermined length L DF (producing a total dispersion D as defined above tot ) to time stretch each spectrally broadened, amplified pulse received at the dispersive medium. The transformed input optical pulses thus exit the dispersive optical medium as time-stretched pulses, where different wavelength components of each pulse exit the dispersive optical medium at different time points, thereby forming a wavelength-scanning optical output.

[0015] Additionally, one or more embodiments of the present invention may take the form of a system that includes a short-pulse seed input having a predetermined value of seed average power and a predetermined value of repetition rate, a pump laser diode that generates a pump signal, a wavelength division multiplexer ("WDM") that combines the seed input and the pump signal, and a dispersive medium having a length L DF , wherein the spectral width of the amplified light source and the repetition rate of the short-pulse seed input match the amount of dispersion provided by the dispersive medium such that the wavelength components of the stretched pulses do not overlap with subsequent pulses.

[0016] In the following discussion, other and further aspects and embodiments of the present invention will become apparent by reference to the related drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Reference is now made to the drawings, where like numerals in several views represent like components:

[0018] Figure 1 is a simplified block diagram of various elements forming the wavelength-scanning light source of the present invention;

[0019] Figure 2 A graph of a time-stretched optical pulse, showing the relationship between the "sweep time" associated with the arrival of different wavelength components within the stretched optical pulse and the "cycle time" associated with the repetition rate of the coherent pulse used as the input to the wavelength-scanning light source;

[0020] Figure 3 is a graph of an exemplary spectrum generated by the wavelength-scanning light source of the present invention, showing the power spectral density (PSD) as a function of wavelength (bottom scale), where the arrival time of the light at the photodetector (defining the "sweep time") is shown along the top scale;

[0021] Figure 4 shows an exemplary embodiment of a fiber-based wavelength-scanning light source formed in accordance with the principles of the present invention; and

[0022] Figure 5 shows an alternative embodiment of a fiber-based wavelength-scanning light source formed in accordance with the principles of the present invention, in which case including a transmission fiber disposed between a doped fiber amplifier and a dispersion fiber output element. DETAILED DESCRIPTION

[0023] Wavelength-scanning light sources for applications such as optical coherence tomography (OCT) typically include tunable lasers. These lasers are known to exhibit high spectral brightness and require only a relatively simple optical design to produce the desired tuning across the available wavelength range. As noted above, conventional devices employ a type of wavelength tuning that involves some mechanical actuation (e.g., mechanical actuation of a movable bandpass filtering element), thus limiting not only the maximum scan speed but also the duty cycle of the device.

[0024] Optical-based "temporal stretching" is an all-optical technique that does not require any type of mechanical tuning control. Instead, optical elements (bulk devices, waveguides, optical fibers, etc.) are used to spread the input pulse as a function of time. That is, the dispersion characteristics of the optical element are used to control the arrival times of the various wavelength components within the optical pulse. This so-called temporal stretching technique (hereinafter sometimes also referred to as "dispersion Fourier transform" (DFT) technique) results in the ability to provide a wavelength-time mapping, thereby producing an effective scan of the wavelength over time. According to the principles of the present invention, the DFT technique can be used in combination with the output from the wavelength-scanning light source of the present invention to provide wavelength scanning over a relatively large spectral range (exceeding 100 nm), with a relatively uniform power distribution across the individual wavelength components (e.g., deviation less than 10 dB), without experiencing the limitations of the scan rate associated with prior art devices with "moving parts".

[0025] Figure 1 is a block diagram showing various elements for forming a wavelength-scanning light source 10 according to the principles of the present invention. Although the elements are shown in this figure as discrete, separate components, it should be understood that each element is preferably formed by a section of optical fiber configured to exhibit characteristics selected to generate a wavelength-scanning output suitable for a particular application with a desired spectral bandwidth and scan rate.

[0026] As Figure 1 shown, the wavelength-scanning light source 10 includes a laser pulse source 12 for providing a sequence of coherent optical pulses at a defined repetition rate (preferably, "ultra-short" pulses having a pulse duration of less than 1 ps). Both the pulse duration and the repetition rate are parameters that can be specifically determined and designed to provide a wavelength-scanning output that meets the requirements of a particular application. The power level of these pulses, as well as their coherence, are other factors important in generating the wavelength-scanning output. At various times, these pulses will be referred to as "seed" pulses, which is a term well known in the art for defining the system input that triggers a series of events for creating the desired output.

[0027] Preferably, a "highly coherent" laser is used as the pulse source 12 because there are applications of wavelength-scanning light sources where the coherence length should be as long as possible. For example, optical coherence tomography (OCT) imaging techniques require a coherence length of at least a certain minimum value (e.g., around 1 mm). "Coherent" means that there is a predictable phase relationship between one or more consecutive pulses.

[0028] Thereafter, the train of pulses output from source 12 is used as the input to doped fiber optical amplifier 14, which is used to inject a controllable amount of gain and spectral broadening into each pulse, thereby creating a spectral bandwidth Δν that defines the upper and lower limits of the wavelength scan range provided by the wavelength scanning light source of the present invention. Additionally, an important aspect is that doped fiber amplifier 14 provides a relatively smooth power distribution across spectral bandwidth Δν. As will be discussed in detail below, while the goal is to provide the widest possible bandwidth, this comes at the cost of increasing the pump power of amplifier 14 to a point where unwanted nonlinear effects degrade the uniformity of the power distribution. One exemplary embodiment described below configures both the pump power and absorption characteristics of the gain fiber to achieve a spectral bandwidth of approximately 130 nm, across which the variation in the power spectral density (PSD) is less than 10 dB.

[0029] Continuing the description Figure 1 of the components, the relatively high-power, spectrally broadened output pulses from doped fiber amplifier 14 are then coupled into dispersion optical element 16. As will be discussed in detail below, the dispersion (D) of this element is a key factor in configuring an element that can sufficiently (in time) separate the wavelength components within the pulse such that a particular wavelength arrives at the output of light source 10 at a sufficiently separated time point (the terms "sufficiently separated", "distinct", etc. as used herein are used to describe the time intervals that allow the associated optical detection device to accurately measure the optical power in each individual wavelength component). Various types of dispersion media can be used to form element 16, including bulk optical nonlinear components, waveguide-based components, and fiber-based components.

[0030] The pulse stretching and "wavelength-time" mapping aspects of the present invention are shown Figure 1 in association with dispersion element 16 in Figure 1 which an input pulse P having a relatively high power level (as it exits amplifier 14) is shown IN . Thereafter, as the pulse propagates along dispersion element 16, the specific dispersion characteristics of the element are used to change the propagation speeds of the different wavelength components within the pulse, resulting in a "time stretching" of the same pulse at the output of wavelength scanning light source 10 (as shown by P Figure 1 in OUT ).

[0031] In most cases, the dispersion of an optical element causes light of longer wavelengths to travel faster than light of shorter wavelengths (variously referred to as "normal" or "negative" dispersion, which is measured in ps / nm-km). However, it is also possible to design a dispersive optical medium to exhibit positive dispersion (sometimes referred to as "anomalous dispersion"), in which light of shorter wavelengths travels faster than light of longer wavelengths. Although either type of dispersive element can generally be used in the wavelength-scanning light source of the present invention, the use of a normal / negative dispersive element is generally preferred and can be formed to exhibit acceptable uniform dispersion across the entire spectral bandwidth Δν. Reference is made to U.S. Patent Application 15 / 970,990, which is assigned to the assignee of the present application and is hereby incorporated by reference, which describes details related to a high "figure of merit" (FOM) optical fiber suitable for use as the dispersive optical element 16.

[0032] Before describing specific embodiments of the present invention, it is useful to consider the relationship between the reciprocal of the repetition rate (also referred to as the "round-trip time") of the seed pulses generated by the pulse source 12 and the "wavelength scan duration". For the purposes of the present invention, the ratio of these two time intervals is defined as the "duty cycle" of the wavelength-scanning light source 10. Figure 2 FIG. illustrates this aspect of the present invention.

[0033] Figure 2 Curve A in FIG. is an exemplary time-stretched output pulse P as it exits the dispersive optical element 16 OUT of the curve. The pulses (shown in an idealized form for illustrative purposes) are plotted to show their power spectral density (PSD) as a function of time. To simplify the following discussion, it is assumed that only a set of three different wavelength components are used to form the "scan" (i.e., the light output from the dispersive optical element 16 is defined to include a set of three spaced-apart wavelength components, here denoted as λ L , λ M and λ S ). As described above, DFT techniques can be used to perform the function of mapping these time-based measurements to actual wavelength values. This mapping can then be used in conjunction with the wavelength-scanning light source 10 to allow an operator of the wavelength-scanning light source to calibrate the arrival times of the time-stretched output pulse sequence from the wavelength-scanning light source 10 to a set of known wavelength values. A set of three time-stretched output pulses are shown as P OUT1 , P OUT2 and P OUT3 in graph A.

[0034] The wavelength scan duration is shown in curve A as the time interval t sweep (i.e., the elapsed time 2Δt). The interval t sweepis a function of the dispersion of the pulses introduced by the dispersive optical element 16; that is, now at the wavelength component λ L at time = t 0 arrival and the wavelength component λ S at time = (t 0 + 2Δt) introduces a time stretch. The "cycle time" t cycle is shown as P in graph A OUT1 rise time and P OUT2 the elapsed time period between the rise times of. The cycle time can also be defined by its reciprocal, the "repetition rate" (f rep ) of the seed pulse. In the embodiments described below, embodiments of the present invention are capable of operating at a repetition rate of 4.7 MHz (cycle time of approximately 200 ns), while maintaining a relatively smooth PSD distribution over a spectral bandwidth Δν of at least 130 nm.

[0035] The duty cycle associated with the exemplary output time-stretched pulse train shown in curve A has a value on the order of about one-half, since t sweep is shown as extending across approximately half of the total cycle time. Although acceptable, since the fiber-based light source of the present invention does not need to be "reset" to an initial state to begin each subsequent scan, it is clear that longer scan times can be used, thereby allowing additional wavelength components to be used in the scan, or allowing a higher resolution output power measurement of the individual wavelength components, or allowing both.

[0036] However, as described above, it is necessary to keep the duty cycle of the wavelength-scanning light source of the present invention less than one. Figure 2 Curve B of shows the situation to be avoided, where the duty cycle has increased to a value greater than one (i.e., t sweep is greater than t cycle ). As shown, this can cause the wavelength components to arrive at the output of the wavelength-scanning light source 10 in a disorderly manner, such that the trailing edge of one pulse overlaps the leading edge of the subsequent pulse. This overlap between the stretched pulses can be attributed to either too high a repetition rate of the seed pulse or too large a total dispersion of the dispersive optical element 16.

[0037] In fact, the preferred embodiments of the present invention are configured to provide a high duty cycle close to one (i.e., t sweep ≈ t cycle ). This is possible because there is no need to reset the mechanical filter assembly before starting a new scan, so once the shortest wavelength of the first pulse has left the source 10, it is ready to transmit the longest wavelength component of the next pulse. Thus, in accordance with the principles of the present invention, a wavelength-scanning light source is provided that can utilize a scan rate that is essentially the same (but not exceeding) the repetition rate of the original seed pulse.

[0038] Figure 3 It is a power spectral density plot of an exemplary wavelength-scanned spectrum that is measured by a photodetector and correlated with specific wavelength values by DFT techniques. Consistent with a conventional illustration, the spectrum is plotted from "short" to "long" wavelength values, showing the power spectral density (PSD) as a function of wavelength (measured in nm). A time scale is shown at the top of the figure, where the "arrival time" of each wavelength component is read from right to left (i.e., higher wavelength components arrive before lower wavelength components). Here, over a spectral bandwidth Δν of about 130 nm, there is a variation in the PSD that remains less than 10 dB, which is more than sufficient to provide a large number of individual wavelength components at substantially the same power level. The time taken for the detector 18 to process this bandwidth is shown as about 100 ns.

[0039] Figure 4 An exemplary wavelength-scanning light source 10A is shown in slightly more detail, which is based on the principles discussed above in connection with Figures 1 - 3 and defines a plurality of parameters that can be configured to obtain a wavelength-scanned output with a defined spectral width and scan rate required for a given application. In particular, all three components (i.e., the pulse source 12, the doped fiber amplifier 14, and the dispersive optical element 16) have parameters that can be specifically selected, designed, or adjusted as needed to meet the requirements of different applications.

[0040] Regarding the specific properties of the pulse source 12, the configuration shown in this embodiment includes a fiber-based laser that is capable of generating coherent ultrafast seed pulses having an average power on the order of 300 μW, a pulse duration of approximately 250 fs, and a repetition rate of 4.7 MHz (which translates to a cycle time on the order of 200 ns). A mode-locked "figure-eight" laser such as that described in U.S. Patent Application No. 16 / 200,810 and assigned to the assignee of the present application is considered an example of a low-noise coherent laser source suitable for this purpose.

[0041] In Figure 4 the specific embodiment shown, the doped fiber amplifier 14 is shown as including a section of erbium-doped gain fiber 40 and a pump source 42 to provide amplification of light at a wavelength of about 980 nm. A wavelength division multiplexer (WDM) 44 is included and is used to direct the seed pulses from the pulsed laser source 12 and the pump light from the pump source 42 into the erbium-doped gain fiber 40.

[0042] In accordance with the principles of the present invention, the doped fiber amplifier 14 is configured to provide spectral broadening of the seed pulse while providing a substantially uniform gain distribution over the created spectral bandwidth Δν. In this embodiment, these characteristics can be achieved by controlling the output power of the pump source 42 in combination with the pump power absorption parameters of the gain fiber 40. In particular, it is known that in some cases, spectral broadening may be related to the pump power level, where as the pump power increases, the increased optical interaction along the gain fiber tends to increase the output wavelength range (i.e., "spectral broadening"). While a wider spectral range means that a larger number of individual wavelength components can be identified and used in the wavelength-scanned output from the source 10A, the increase in pump power necessary to achieve this can also lead to amplification of unwanted noise components contained within the propagating wave or generated during the amplification process itself.

[0043] Accordingly, an important aspect of the present invention relates to determining an acceptable amount of gain over a specific spectral bandwidth Δν that is useful for a given application without amplifying noise components outside of that range. In fact, there is an upper limit to the amount of gain that should be provided by the doped fiber amplifier 14, where it has been found that excessive gain can cause harmful nonlinear effects such as self-phase modulation (SPM), cross-phase modulation (XPM), Raman scattering, etc. (collectively referred to as "noise"). Thus, an "acceptable amount" of gain is associated with ensuring that the doped fiber amplifier 14 operates in a "low-noise" state. The specific range of acceptable values is discussed below in connection with Figure 5 the embodiments.

[0044] Continuing the description of the light source 10A as Figure 4 shown, the amplified and spectrally broadened pulse generated by the amplifier element 14 is then coupled into the dispersive optical element 16, which in this case comprises a section of dispersive fiber 160 shown as having a defined length L DF . As described above, each pulse passing through the dispersive fiber 160 is "stretched" in time such that different wavelength components within the pulse arrive at the output of the light source 10A at measurably different time points.

[0045] In some embodiments of the present invention, due to the reasons discussed above in connection with Figure 2 , the length L DF of the dispersive fiber 160 can be optimized to provide a duty cycle close to one. In fact, it has been found that a duty cycle close to one provides improved spectral resolution for a given detection bandwidth (which is typically defined as the combination of the photodetector response time and the digitizer bandwidth). The approximate value of the optimized length of the dispersive fiber 160 (L DF,opt ) can be obtained from the following formula:

[0046]

[0047] where D avg is the average dispersion value of the dispersion fiber 160 over the bandwidth in question, and the other terms in the relationship are defined as above.

[0048] Figure 5 Another embodiment of the present invention is shown, referred to as the wavelength-scanning light source 10B. In this particular embodiment, an additional section of optical fiber is included within the wavelength-scanning light source. In particular, this section of optical fiber 50 is shown disposed between the output of the amplifier 14 and the input of the dispersion fiber 16. The optical fiber 50 is sometimes also referred to as the "transmission" fiber and can be included in applications where the doped optical fiber amplifier 14 cannot be positioned relatively close to the dispersion element 16, or in applications where additional spectral broadening is required before introducing the pulse into the dispersion medium. Additionally, it is contemplated that an additional section of standard single-mode optical fiber is included between the gain fiber 40 and the dispersion fiber 160, which allows the use of a pair of fusion connections (as Figure 5 shown by X in) to maintain an effective coupling with low power loss between the core regions of the erbium-doped gain fiber 40 and the dispersion fiber 160. In an exemplary embodiment, the optical fiber 50 can include a section of single-mode optical fiber that is fused to the ends of both the erbium-doped gain fiber 40 and the dispersion fiber 160.

[0049] For Figure 5 the particular embodiment shown, the laser pulse source 12 is specifically shown as an 8-shaped fiber-based laser (e.g., as disclosed in the above-referenced U.S. Patent Application 16 / 200,810), which includes a unidirectional optical fiber loop 60 and a bidirectional ring "mirror" 62, where an optical coupler 64 provides signal coupling between the two loops. An output coupler 66 is used to direct a portion of the signal to circulate around the unidirectional optical fiber loop 60 (including mode-locked optical pulses) along the output path and into the doped optical fiber amplifier element 14.

[0050] Here, the doped optical fiber amplifier element 14 is shown as using a section of Er-doped optical fiber 40 having a nominal absorption (of the propagating pump wave) on the order of about 27 dB / m. A pump source 42 is shown as providing a pump beam at a wavelength of 976 nm and is set to operate at a pump power of 250 mW in this case. For this particular combination of amplifier parameters, when considering using a seed pulse with an input pulse energy of about 60 pJ (i.e., a power of 300 μW at a 4.7 MHz repetition rate), it has been found that using a length L ErAn erbium-doped gain fiber 40 on the order of about 2.5 m provides a relatively uniform power spectral density (PSD) over the spectral band of interest. In particular, for this set of parameters, it has been found that the output pulses from the doped fiber amplifier 14 exhibit a pulse energy of about 2 - 4 nJ (corresponding to an output power in the range of about 10 - 20 mW at a 4.7 MHz repetition rate), with a PSD of less than 10 dB over a spectral range greater than 130 nm.

[0051] It should be understood that the specific values described above for the design of the pulse source 12 and the doped fiber amplifier 14 are merely exemplary values that are coordinated in a manner useful for generating a wavelength-scanned output in an exemplary configuration incorporating the dispersion fiber 160, as will now be continued with reference to Figure 5 discussed.

[0052] As described above, a section (single-mode) transmission fiber 50 is included in the fiber-based wavelength-scanning light source 10B as Figure 5 shown, where these high-power output pulses from the doped fiber amplifier 14 are coupled into the transmission fiber 50 for transmission through and subsequent coupling into the dispersion fiber 160. In the Figure 5 specific embodiment shown, the dispersion fiber 160 is formed to have an average dispersion value (D avg ) on the order of about -75 ps / nm / km. Utilizing this property, it has been found that a dispersion fiber with a length L DF of 7 km provides a Figure 3 wavelength-scanned output of the form shown, having a spectral bandwidth Δν of approximately 130 nm and a PSD variation of less than 10 dB over this bandwidth. As described above, an important factor in configuring the dispersion fiber 16 is to provide a controlled amount of dispersion across the entire spectral bandwidth. An exemplary dispersion fiber known as a "high quality factor" fiber exhibits relatively linear dispersion characteristics. U.S. Patent Application 15 / 970,990, entitled "Optical Fiber with Specialized Figure-of-Merit and Applications Therefore," assigned to the assignee of the present application, includes a description of a particular type of dispersion fiber that can be accepted for use in a fiber-based wavelength-scanning light source formed in accordance with the principles of the present invention.

[0053] Thus, a wavelength-swept light source can be constructed by combining a pulsed laser source with an appropriate amount of dispersion for temporal stretching. The dispersion is preferably well-matched to the bandwidth and repetition rate of the input light source such that the wavelength components of the stretched pulses do not overlap with subsequent pulses. It is generally desirable to have as much output power as possible and as wide a wavelength range as possible, while maintaining a smooth distribution of power over the available spectral range and low levels of power fluctuations from one pulse to the next.

[0054] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.

Claims

1. A method for generating a wavelength-scanned optical output from a light source, comprising: providing a series of optical input pulses at a predetermined repetition rate; amplifying and spectrally broadening each optical input pulse of the series of optical input pulses within a fiber-based optical amplifier such that each amplified and spectrally broadened pulse spans a predetermined spectral bandwidth Δν, which spectral bandwidth Δν defines the upper and lower wavelength limits of the wavelength-scanned optical output; and Pass each amplified and spectrally broadened pulse through a dispersive optical medium having a predetermined average dispersion D per unit length avg and a predetermined length L DF , thereby defining D avg *L DF as the total dispersion D tot , the dispersive optical medium performing temporal stretching on each propagating pulse according to D tot such that the respective wavelength components exit the dispersive optical medium at different time points, thereby defining a time interval t between a first-arriving wavelength component and a last-arriving wavelength component sweep , the predetermined total dispersion D tot being selected such that t sweep is less than t cycle , such that the temporally stretched pulses exiting the dispersive optical medium do not overlap in time, where t cycle is the reciprocal of the predetermined repetition rate of the series of optical input pulses.

2. The method according to claim 1, wherein the optical input pulses have a duration of less than 1 ps.

3. The method according to claim 1, wherein the predetermined repetition rate of the optical input pulses is not less than 2 MHz.

4. The method according to claim 1, wherein the length of the dispersive optical medium is chosen to be substantially equal to the reciprocal of the product of the pulse repetition frequency, the average dispersion D avg and the spectral bandwidth.

5. A wavelength-scanning light source, comprising: a laser pulse source configured to generate a train of optical input pulses exhibiting a predetermined repetition rate; a pump beam source configured to generate an optical pump beam at a pump wavelength suitable for applying gain to each pulse forming the train of optical input pulses; a doped fiber amplifier for receiving as separate inputs a sequence of optical input pulses from the laser pulse source and the optical pump beam from the pump beam source, the doped fiber amplifier producing a spectrally broadened output pulse having a minimum variation in power spectral density over a predetermined bandwidth Δν, which predetermined bandwidth Δν defines the upper and lower wavelength limits of the wavelength-scanned optical output; and A dispersive optical medium is coupled to the output of the doped optical fiber amplifier, the dispersive optical medium having an average dispersion per length D avg and the predetermined length L DF , thus providing a value defined as D avg *L DF The total dispersion D tot , where each pulse in the output pulse train is tot The time stretching is performed so that each wavelength component of each pulse leaves the dispersive optical medium at a different time point, thereby defining a time interval t from the first arriving wavelength component to the last arriving wavelength component. sweep , the total dispersion D tot is chosen so that t sweep Less than t cycle , so that the time-stretched output pulses from the dispersive optical medium do not overlap in time, where t cycle is the inverse of the predetermined repetition rate of the optical input pulse train.

6. The wavelength-scanning light source according to claim 5, wherein, The length L of the dispersive optical medium DF is estimated to be: where t cycle is the pulse interval in time, f rep is the pulse repetition rate, D avg is the average dispersion of the dispersion fiber at the bandwidth value, and Δν is the spectral bandwidth of the wavelength-swept light source.

7. The wavelength-scanning light source according to claim 5, wherein the doped fiber amplifier comprises an erbium-doped fiber amplifier that utilizes a pump source to provide a beam at a nominal wavelength of about 980 nm and a pump power of at least 200 mW.

8. The wavelength-scanning light source according to claim 5, wherein the dispersive optical medium comprises a section of dispersion fiber.

9. The wavelength scanning light source according to claim 8, wherein said section of dispersion compensating fiber exhibits an absolute average dispersion |D| of at least 75 ps / nm-km and has a length L in the range of about 7 km to about 10 km. avg | and has a length L in the range of about 7 km to about 10 km. DF .

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