Figure-eight laser

Abstract

A figure-eight laser is constructed that maintains mode-locked operation with the gain of the bidirectional nonlinear amplifier loop alone while the gain of the unidirectional loop can be removed. Simplified self-starting is achieved by turning off the pump power in the unidirectional loop, and overall control of pulse characteristics is possible by controlling the gain of the bidirectional loop in the absence of pump power in the unidirectional loop.

Description

[Technical Field]

[0001] This patent application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 63 / 070,062, filed August 25, 2020.

[0002] This specification relates to a mode-locked fiber laser. [Background technology]

[0003] Ultrashort-pulse fiber lasers are compact and stable, playing a key role in many applications, including medical imaging and other major industries, including electronics, semiconductors, and microtechnology. Ultrashort-pulse lasers are useful for manufacturing components with unprecedented quality, precision, and speed. Due to the short pulse duration, laser energy can be input into the material in a shorter time than heat can deposit, thus preventing thermal damage to the component. In these applications, stability against environmental perturbations such as temperature fluctuations, stress, and humidity is a crucial aspect. By using polarization-maintaining (PM) fiber and PM components, the linear polarization state can be maintained during propagation, thereby achieving environmentally stable ultrashort-pulse fiber lasers.

[0004] Mode-locking a laser is a well-known method for generating ultrashort laser pulses. This method is based on the generation of many frequencies with synchronized phase relationships between ultrashort laser pulses within a defined pulse duration. To achieve mode-locking, fiber lasers utilize, for example, real and artificial saturable absorber (SA) devices. Real SA devices include semiconductor SA mirrors (SESAMs), single-walled carbon nanotubes, and graphene. However, these devices suffer from short operating lifetimes and cannot tolerate high-energy pulse operation. Artificial SA devices are more robust mode-locking mechanisms that can provide short pulse durations and long-term stable operation when implemented in an all-PM fiber laser cavity. Examples of artificial SA mechanisms include nonlinear polarization rotation (NPR), nonlinear optical loop mirrors (NOLMs), and nonlinear amplifying loop mirrors (NALMs). Despite the advantages of artificial SA devices, these devices have their own limitations.

[0005] The NPR mechanism limits its use to non-PM cavities and suffers from environmental perturbations and mechanical vibrations. As a result, output pulse characteristics from these systems can be irreproducible. NOLM and NALM devices operate using the same general principle: the Sagnac interferometer. These devices are constructed from fused couplers whose output ports are spliced ​​together to form a loop. In a loop, light propagates with unequal intensity in the counterclockwise and clockwise directions. This can be achieved by unequal coupler splitting ratios (NOLM) or by including an in-line fiber amplifier near one of the coupler's ports (NALM). In both cases, the other two ports of the fused coupler are spliced ​​to the end of an optical fiber train to form a unidirectional loop, which is then connected to a bidirectional nonlinear loop mirror by the fused coupler. Compared to NOLM, NALM improves the laser's self-starting mode-locking capability by creating a strong loop asymmetry.

[0006] For a long time, figure-eight fiber lasers (based on NOLM or NALM) lacked reliable self-starting mode-locking unless an additional intra-cavity amplitude modulator was added to the loop mirror. Without the amplitude modulator, reliable self-starting could only be achieved by optimizing the cavity losses and the coupling ratio of the four-port coupler connecting the two loops (unidirectional and bidirectional) of the figure-eight fiber laser. Optimizing the laser design would imply careful selection of each and every component, including the laser cavity, which creates significant challenges for industrial applications. Once single-pulse operation is achieved, the pulse parameters (i.e., energy, spectral bandwidth, and duration) can be easily adjusted by varying the pump power of the laser's unity-gain element.

[0007] The problem of reliable self-starting has been solved in NALM-based fiber lasers by incorporating an additional gain element in the unidirectional loop (i.e., two gain elements within the laser cavity). However, solving the self-starting reliability problem has come at the expense of increasing the complexity of the system design. The addition of a second independent amplification stage to the unidirectional loop, which allows for tuning of nonlinearities and compensation for losses within the laser cavity, has been shown to significantly expand the possible range of generated pulse parameters (i.e., energy, spectral bandwidth, duration, and degree of coherence).

[0008] The first demonstration of a figure-eight fiber laser based on NALM, with one independent gain element in each loop, was constructed exclusively with fiber having normal dispersion. In this single laser cavity design, pulses were generated over an order of magnitude of energy range and pulse duration, and with different degrees of coherence, by varying the ratio of the pump powers of the two gain elements. Several reports have investigated the effect of the pump power of each of the two gain elements on the output pulse parameters.

[0009] Therefore, the development of new methods for controlling the parameters of generated pulses was required for these complex laser architectures. Machine learning and electronic control of the ratio of pump powers have been proposed as methods for controlling the pulse duration, pulse energy, and spectral bandwidth of generated laser pulses. Using these methods, the laser system self-regulates its parameters by incorporating an optical feedback loop that links laser performance with two or more variable cavity parameters. However, machine learning requires a large set of complex algorithms and diagnostic monitors to provide input for the feedback loop, and using the ratio of pump powers creates control issues over long-term laser operation because the two pump sources can degrade differently over time. Both methods present limitations and problems for industrial applications.

[0010] Another major challenge in NALM-based and NOLM-based figure-8 fiber lasers using all-normal dispersion fibers is establishing a low-noise, stable pulse regime. Dispersion-managed figure-8 fiber lasers composed of anomalous dispersion and normal dispersion fibers present an attractive design for generating pulses with excellent ultralow noise performance. Depending on the net cavity dispersion, numerous pulse evolutions can occur, which result in very different output pulse characteristics.

[0011] Figure-eight fiber lasers based on the NPR, NOLM, and NALM mechanisms have been demonstrated with large net normal cavity dispersion, large net anomalous cavity dispersion, and net cavity dispersion approaching zero. The paper demonstrates that the best performance of the output pulses, in terms of large spectral bandwidth and small timing jitter, can be achieved by managing the net cavity dispersion of the fiber laser to approach zero. Pulses generated in this cavity dispersion regime are called stretched pulses, meaning they can be linearly chirped and compressed to obtain near-transform-limited pulses with short durations. Summary of the Invention [Means for solving the problem]

[0012] In prior art figure-eight lasers, reliable mode-locked operation requires gain in both loops. Applicants have constructed a figure-eight laser that can eliminate the gain in the unidirectional loop while maintaining mode-locked operation with gain only in the bidirectional nonlinear amplification loop.

[0013] Applicant has discovered that a robust and stable monopulse state of operation can be reliably achieved using a constant, predetermined pump current adjustment without performing any iterative search or feedback-based adjustment of the pump source to establish a stable monopulse state of operation.

[0014] Furthermore, with the pump power providing the gain of the bidirectional nonlinear amplification loop alone, the pulse characteristics may be easily controlled by measuring the output power and adjusting the pump power of the gain element of the bidirectional loop.

[0015] In some embodiments, a mode-locked, figure-eight fiber laser is provided, comprising: a fused fiber coupler for connecting two fiber loops; a bidirectional loop connected to the coupler and having a gain element and a first pump source; and a unidirectional loop connected to the coupler and having a gain element, a second pump source, an isolator, a filter, and an output coupler. The laser is configured to operate in a stable single-pulse state of operation with the second pump at zero power. A controller coupled to the first pump source and the second pump source may be configured to drive both the first pump source and the second pump source at high power to initiate high-order multipulse state operation, then drive the first pump source at a lower power to transition to low-order multipulse state operation, and then drive the second pump source at zero power to transition to the stable single-pulse state of operation.

[0016] The unidirectional loop and the bidirectional loop may be constructed from PM fiber and PM components.

[0017] The laser may include a length of optical fiber having group velocity dispersion of opposite sign to other fibers in the laser for dispersion management, and the monopulse state of operation provides a stretched pulse in use.

[0018] The isolator, the filter and the output coupler may comprise an optical circulator and a chirped fiber Bragg grating.

[0019] The gain element of the bidirectional loop may include a section of rare-earth doped fiber, for example, ytterbium doped fiber.

[0020] The gain element of the unidirectional loop may include a section of rare-earth doped fiber, for example, ytterbium doped fiber.

[0021] Net cavity group velocity dispersion is -0.16ps 2 to +0.15ps 2 The laser may be configured to be in the range of

[0022] The laser may be configured such that the spectral bandwidth of the filter (or CFBG) is at least 7 nm.

[0023] The laser may be configured to produce output pulses in the single-pulse state of operation having a duration at FWHM of less than 15 ps.

[0024] The controller may be configured to vary the power levels of the first pump source and the second pump source in predetermined increments with a wait time between successive increments as the power levels change from the high power to initiate a high-order multipulse state of operation, then to drive the first pump source at a lower power for a transition to a low-order multipulse state of operation, and then to drive the second pump source at zero power for a transition to a stable single-pulse state of operation.

[0025] The controller may be configured to drive the second pump source at a constant low level of power after the laser has transitioned to a stable single-pulse state of operation to provide greater output power, longer pulse duration, and greater spectral bandwidth from the laser.

[0026] The laser may further include an output power sensor, and the controller may be configured to drive the first pump source at a level to stabilize the output power of the laser in response to the output power sensor.

[0027] The laser may be configured to generate output pulses in the single-pulse state of operation having a repetition rate greater than 5 MHz.

[0028] The laser may be configured to produce output pulses in the single-pulse state of operation having a FWHM spectral bandwidth greater than about 1 nm.

[0029] The laser may be configured to generate output pulses having an average output power greater than about 1 mW during the single-pulse state of operation.

[0030] In some embodiments, a method for generating a stable train of laser pulses is provided, a fused fiber coupler for connecting two fiber loops; a bidirectional loop having a gain element and a first pump source connected to the coupler; A unidirectional loop is connected to the coupler and includes a gain element, a second pump source, an isolator, a filter, and an output coupler. providing a figure-eight laser having: driving both the first pump source and the second pump source at high power to initiate high-order multi-pulse state operation; driving the first pump source at a low power to transition to a low-order multipulse state of operation; Driving the second pump source above at zero power for transition to a stable single-pulse state of operation Includes. It will be appreciated that the figure eight laser can provide simplified self-starting and that by controlling the gain of the bidirectional loop the overall pulse characteristics can be controlled.

[0031] The present invention will be better understood from the following detailed description of an embodiment of the invention, taken in conjunction with the accompanying drawings, in which: [Brief explanation of the drawings]

[0032] [Figure 1] 1 is a schematic diagram of a figure-eight laser according to a first embodiment. FIG. [Figure 2] FIG. 1 is a sequence diagram of laser mode-locking startup control according to an embodiment. [Figure 3] FIG. 10 is a schematic diagram of a figure-eight laser according to a second embodiment. [Figure 4] Graph showing pump power combinations at three different stages during startup. [Figure 5] 5 is a graph showing OSA spectra for the three stages of FIG. 4. [Figure 6A] FIG. 5 shows the pulse intensity temporal shape for one stage of FIG. 4. [Figure 6B] FIG. 5 shows the pulse intensity temporal shape for one stage of FIG. 4. [Figure 6C] FIG. 5 shows the pulse intensity temporal shape for one stage of FIG. 4. [Figure 7A] Figure 5 shows the pulse spectrum shape for one stage of Figure 4. [Figure 7B] Figure 5 shows the pulse spectrum shape for one stage of Figure 4. [Figure 7C] Figure 5 shows the pulse spectrum shape for one stage of Figure 4. [Figure 8] Figure 3 is a plot of the percent change in duration, bandwidth and output power as a function of pump power for the gain segment in the bidirectional loop for the laser. [Figure 9] FIG. 10 is a schematic diagram of a figure-eight laser according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0033] The embodiments described below provide a passively mode-locked fiber laser that reliably self-starts with controllable output pulse operation. The fiber laser consists of a figure-eight shaped, all-fiber laser ring cavity, a free-space void. In one embodiment, the laser is constructed from optical components with opposite group-velocity dispersion and has near-zero net cavity dispersion (a dispersion-managed cavity). In another embodiment, the laser cavity contains only components with normal dispersion (an all-normal dispersion cavity). The ring cavity in both laser embodiments contains two loops. The first loop is called the nonlinear amplification loop, and the second loop is called the unidirectional loop. The fiber loops are connected using a fused coupler. The laser utilizes two amplifiers for self-starting: one amplifier in the unidirectional loop and the other in the nonlinear amplification loop. Control methods are used to reliably initiate mode-locking of the laser, maintain mode-locked operation under changing conditions, and control the laser output pulse parameters.

[0034] A control method using a novel continuous startup and novel optimization of the figure-eight fiber laser architecture results in reliable self-startup and simplifies control of the output pulse parameters of figure-eight fiber lasers. This method can be used to control dispersion-managed, all-normal-dispersion, or all-anomalous-dispersion figure-eight fiber lasers. Using the above control method, the laser can establish stable single-pulse operation using only the amplifier installed in the nonlinear amplification loop while the other amplifiers are turned off. Operating a figure-eight fiber laser based on NALM (containing two amplifiers) with only a single amplifier operating simplifies control of the laser's output pulse parameters. By monitoring the output optical power, the driver current of the amplifier in the nonlinear amplification loop can be electronically controlled to maintain the power inside the cavity within a certain rate of change. By controlling the amplifier drive current, the nonlinearity of the amplification loop mirror, which is important for maintaining laser mode-locking, is also maintained.

[0035] Referring to the drawings, Figure 1 illustrates a schematic example of a dispersion-managed, modelocked, figure-eight fiber laser providing stretched pulses. The fiber laser is shown to include a nonlinear amplification loop 1 and a unidirectional loop 2. A 2x2 central coupler 3 is included and utilized to create a path between loops 1 and 2.

[0036] The unidirectional loop 2 is shown to include an amplifier 4. The amplifier compensates for losses experienced by pulses propagating within the laser cavity. Amplifier 4 can be a semiconductor-based optical amplifier or a fiber-based optical amplifier. An output coupler 5 is connected to the output end of amplifier 4, coupling out a portion of the optical power circulating within the laser cavity. This portion of the output power is photodetected at detector 28 and used to form the feedback loop necessary to control the laser's output pulse parameters. Power can alternatively be measured at a location separate from the output. A spectral bandpass filter (BPF) 6 is placed after the output coupler to reshape the pulses propagating within the cavity and determine the laser's center wavelength. An optical fiber 7 can be placed in the loop, preferably after BPF 6, to manage the net dispersion of the laser cavity. If optical fiber 7 is not used in the loop, laser 10 is an all-normal dispersion fiber laser, not a dispersion-managed fiber laser. Optical fiber 7 has a group velocity dispersion magnitude opposite to that of the other cavity fibers. Components 5, 6 and 7 can be placed either before or after amplifier 4. An optical isolator 8 can be placed at the output of fiber 7, which ensures unidirectional operation and blocks pulses propagating in the opposite direction.

[0037] The nonlinear amplification loop 1 is shown to include an amplifier 9. The remainder of the nonlinear amplification loop 1 can be formed from normal dispersion fiber. This fiber loop is responsible for converting the continuous wave radiation into pulsed radiation and for initiating the mode-locking mechanism.

[0038] The novel method used to reliably initiate laser mode-locking and control the output pulse parameters of this laser is shown in Figure 2. Controller 29 has the ability to control the drive currents of amplifiers 4 and 9 of the unidirectional and nonlinear amplification loops, respectively. To initiate the mode-locking mechanism, controller 29 does not need to use any monitors to control the operating parameters. The reason no monitors are required during laser startup is that the laser mode-locking mechanism is not very sensitive to the gain of both amplifiers 4 and 9. There is a wide range of gain values ​​over which the laser can self-start. Adding monitors would be possible, but would only add to the control complexity and time required to start the laser. Instead, the current drivers of both amplifiers can be varied according to preprogrammed current values.

[0039] The controller 29 may be any suitable processor (e.g., a microcontroller, a computer) running suitable software, or the controller may be implemented using circuitry such as a programmable logic controller (PLC), FPGA, or hard-wired analog and / or digital circuitry.

[0040] For example, when the oscillator is started, the controller 29 can simultaneously ramp both current drivers to their respective preprogrammed startup current values ​​of 1, as shown in stage 1 of FIG. 2. The driver current values ​​for amplifiers 4 and 9 do not necessarily need to be the same. An additional delay may be added between starting the current drivers for each amplifier. In subsequent stages, the controller changes the drive currents for both amplifiers in a sequence following the arrows shown in FIG. 2. The wait time between stages may also be different from the illustrated wait time of 3 seconds. If the startup state consists of only three stages as shown in FIG. 2 (or up to N stages, if required based on the laser design), the drive current value 2 for amplifier 9 (or stage N-1) is important for turning off the drive current for amplifier 4 in stage 3 (or stage N) and still obtaining stable single-pulse operation. If the drive current value 2 for amplifier 9 is too high in stage 2, the laser may continue to operate in the multi-pulse regime even after the drive current for amplifier 4 is turned off in stage 3. If the amplifier 9 current value 2 is too low, the laser may not be able to operate in stable single-pulse operation after the amplifier 4 drive current is turned off in stage 3, and only CW radiation will circulate within the cavity.

[0041] After completing the startup stage, the controller waits, for example, 3 seconds before monitoring the output power, using, for example, a photodiode. This power is compared to a predetermined reference value, and the pump power of the amplifier in the nonlinear amplification loop is increased or decreased if the power differs from the reference value. In this way, the laser generates output pulses that are stable (power, bandwidth, duration) against changing environmental conditions, as discussed below.

[0042] Figure 3 schematically illustrates another embodiment of a dispersion-managed, mode-locked, figure-eight fiber laser that produces stretched pulses. The fiber laser is shown to include a unidirectional loop 12 and a nonlinear amplification loop 11. A 2x2 central coupler 13 is included and utilized to create a path between loops 11 and 12. In this figure-eight fiber laser embodiment, all fibers and components are polarization-maintaining. The overall PM architecture helps ensure that the laser produces pulses that are stable against environmentally changing conditions when the power control feedback loop is enabled.

[0043] The unidirectional loop 12 is shown to include a section of rare-earth doped gain fiber 14, with an optical pump beam 16 being introduced into the gain fiber 14 via a wavelength division multiplexer (WDM). Ytterbium is typically utilized as the rare-earth dopant for the gain fiber, where a pump beam of 976 nm wavelength is known to provide the inversion necessary to introduce gain for the optical signal propagating through the gain fiber. However, the gain fiber 14 can utilize other rare-earth dopants (including Er, Tm, Ho), where each provides gain over a different optical wavelength range and therefore produces output pulses at different wavelengths.

[0044] An optical circulator 18 is placed at the output of the gain fiber 14, ensuring unidirectional operation and blocking counter-propagating pulses. A chirped fiber Bragg grating (CFBG) is connected to port 2 of the circulator 18 and is used to control the net dispersion of the cavity and also acts as a spectral bandpass filter. The dispersion of the CFBG 21 is one operating parameter that can be controlled based on the length of the laser cavity to maintain the net dispersion within a specified range. At the output end of the CFBG, a portion of the output power can be photodetected and used to form the feedback loop necessary to control the laser's output pulse parameters. As mentioned above, although the photodetector for feedback purposes can be located anywhere within the laser, it is convenient to locate the photodetector at the output. As discussed below, maintaining the lengths of fibers 17, 19, 22, and 27 within a specified range is a design parameter that can be controlled to achieve self-starting, resulting in stable single-pulse operation.

[0045] The nonlinear amplification loop 11 is shown to include a section of rare-earth doped gain fiber 23, with an optical pump beam 25 introduced into the gain fiber 23 via a WDM 24. The length of the fiber 26 is one design parameter that can be controlled to provide self-starting operation.

[0046] In one example of a dispersion-managed figure-eight fiber laser, laser 20 is an ytterbium-doped fiber oscillator delivering pulses at a wavelength of approximately 1030 nm ± 1 nm. The laser delivers output pulses having a FWHM duration of approximately 2.4 ps at a repetition rate of approximately 25 MHz. The pulses have a FWHM spectral bandwidth of approximately 14.7 nm. The average output power of the pulse train is approximately 7.9 mW. Preferably, the laser is configured to achieve pulse durations shorter than approximately 15 ps and average power greater than 1 mW at a repetition rate greater than approximately 5 MHz (more preferably greater than approximately 10 MHz) and a FWHM spectral bandwidth greater than approximately 7 nm. CR13 has a 60 / 40 coupling ratio. As those skilled in the art will recognize, this coupling ratio may vary from the exemplary 60 / 40, although 60 / 40 is more efficient than 50 / 50 or 80 / 20. The length of gain fiber 14 may be approximately 0.5 m. Circulator 18 is a low-loss circulator with a total insertion loss of 1 dB. This circulator is used to reduce the total loss within the cavity. The loss of the circulator is one operating parameter that can be controlled to achieve self-starting operation. The loss is preferably less than 2 dB. CFBG 21 has anomalous dispersion of D2 = 0.42 ps / nm and a reflectivity of 0.37. The length of fiber 17 may be approximately 0.4 m, the length of fiber 19 may be approximately 0.6 m, the length of fiber 22 may be approximately 1.8 m, the length of fiber 27 may be approximately 0.5 m, the length of gain fiber 23 may be approximately 0.5 m, and the length of fiber 26 may be approximately 1.5 m.

[0047] For a given layout of the nonlinear amplification loop and CR, it has been found that there is a set of operating parameters for the unidirectional loop that reliably achieves self-starting of the figure-eight fiber laser, resulting in a stable single-pulse operating regime. As mentioned above, the CFBG21 acts as a spectral bandpass filter. It has been found that the spectral bandwidth of the CFBG21 affects the self-starting of the fiber laser. The relatively narrow bandwidth of the CFBG21 increases intracavity losses, and therefore only CW radiation circulates within the laser cavity. The minimum acceptable bandwidth has been found to be in the range of 7 nm. To obtain stretched pulses from the laser, the dispersion of the CFBG21 must be adjusted so that the net cavity dispersion is -0.16 ps 2 to +0.15ps 2 The voltage is controlled to be within the range of .

[0048] According to the present invention, the lengths of fibers 17, 19, 22, and 27 are parameters that can be controlled to promote the desired self-starting behavior of the figure-eight laser cavity. The length of each of these fibers can be adjusted to modify the amount of dispersion and nonlinearity introduced by this section. It has been found that for fiber 17 lengths ranging from 0.1 to 2 m, the laser was able to self-start into a stable stretched-pulse regime. Once the laser is operating in the single-pulse regime, pump source 16 may be turned off, and only pump source 25 needs to remain operating. However, if the length of fiber 17 is selected to be longer than 2 m, the laser was able to self-start into a stable stretched-pulse regime, but both pump sources 16 and 25 need to remain operating in the single-pulse regime. Additionally, fiber 19 must be longer than 0.35 m to achieve reliable self-starting. Fiber 19 may be as long as 7 m, depending on the desired repetition rate for this laser. It was found that for fiber 22 lengths ranging from 1.5 m to 3.5 m, the laser was able to self-start into a stable stretched pulse regime. The preferred length of fiber 22 depends on the fiber mode field diameter and the laser wavelength. For lengths longer than 3.5 m, the laser does not self-start, and only CW radiation circulates within the cavity. For lengths shorter than 1.5 m, the laser operates only in the multipulse operating regime, not short-pulse operation. It was also found that for fiber 27 lengths in the range between 0.5 m and 1.5 m, the laser reliably self-starts. For lengths longer than 2 m, the laser cannot self-start. The lengths given herein are given by way of example and are suitable for operation at wavelengths in the 1 micron range, within the gain bandwidth of Yb. For different wavelengths, the length may need to be adjusted. As will be appreciated by those skilled in the art, the lengths of fibers 17, 19, 22 and 27 may differ from the values ​​stated above if special fibers (high or low nonlinearity PM fibers) are used instead of standard PM optical fibers.However, it is preferred that the following condition be satisfied: (γ×L). special fiber =(γ×L) used fiber , where γ is the waveguide nonlinearity and L is the length of the fiber.

[0049] The length of the gain fiber 14 is another parameter that can be controlled to promote the desired self-starting behavior of the figure-eight laser cavity. The length of the gain fiber 14 can be adjusted to modify the amount of dispersion and nonlinearity introduced by this section. It has been found that for fiber lengths in the range of 0.3 to 0.7 m, the laser can self-start into a stable stretched-pulse regime, providing preferably stretched pulses. For lengths shorter than 0.3 m, the laser does not self-start, and only CW radiation circulates within the cavity. For lengths longer than 0.7 m, the laser operates only in the multi-pulse operating regime, not in the short-pulse operation regime. With the gain fiber 14 within the specified length range, it has been found that during operation in the single-pulse regime, the pump 16 can be kept at a carefully controlled level or, advantageously, turned off, thereby simplifying control of the laser output pulse parameters.

[0050] Additionally, the initial optical power levels of pump source 16 and pump source 25 and the sequence for decreasing the optical power levels of pump source 16 and pump source 25 have been found to be additional parameters that result in the desired self-starting behavior of the figure-eight fiber laser. For this example, starting first pump source 25 at a pump power in the range of approximately 350–450 mW and then starting pump source 16 at a pump power in the range of approximately 170–270 mW has been found to result in self-starting of the figure-eight laser cavity into a high-order multi-bounce stretched pulse (more than six-bounce states) operating regime. Starting the laser above the power threshold level for the single-pulse operating regime has been found to be essential for robust self-starting under different environmental conditions. In the second stage of this sequence, pump source 16 is maintained at the same power level as in the previous stage, and pump source 25 is reduced to a power in the range of approximately 160–220 mW. At this stage, the laser switches to a low-order multi-bound (mostly dual- or triple-bound) pulsed operating regime. In the third stage of this sequence, pump source 16 is turned off, and pump source 25 is maintained at the previously set power level. At this stage, the laser switches to a single-pulse operating regime. At this stage, having pump source 25 at a power level lower than the 160-220 mW range results in a loss of laser mode-locking, and only amplified optical noise is emitted at the output of the laser instead of optical pulses. Additionally, having pump source 25 using a power level higher than the 160-220 mW range results in a sustained low-order multi-bound stretched pulse regime.

[0051] 4 is a graph illustrating an example of successive stages of pump power reduction for pump source 16 and pump source 25. It is emphasized here that the specific sequence described above should not be construed as limiting the present invention. Those skilled in the art will appreciate other pump power adjustment sequences from the description provided herein. While a single-pulse operating regime can also be achieved with both pumps operating, the figure-eight laser described herein advantageously has the ability to operate in the single-pulse regime with only the bidirectional loop amplifier powered.

[0052] Figure 5 is a graph illustrating exemplary measured spectra of three different operating pulse regimes obtained from an experimental version of the apparatus in Figure 3. These spectra were generated with the three pump power reduction stages discussed above and shown in Figure 4. The first pulse regime, identified as graph H in Figure 5, is associated with both pump power levels higher than those required for a stable single-pulse regime. High pump power in the fiber laser cavity enables the generation of high-order multi-bounce pulses, and amplified spontaneous emission (ASE) noise is also enhanced under strong pumping in the ytterbium fiber amplifier. At large pump power levels, ASE is sufficient to affect the phase synchronization between adjacent bound pulses. Variations in relative phase synchronization result in the interaction of adjacent pulses, which weakly modulates the spectrum, as shown in graph H in Figure 5.

[0053] Figure 6(A) is a graph of the autocorrelation trace of the pulse structure generated at the pump power level associated with graph H in Figure 5. The graph reveals the presence of closely spaced eight-bound pulses (tight bounds) involved in the spectral generation. Figure 7(A) shows the corresponding RF spectrum of a high-order multi-bound pulse. The results reveal that the signal-to-noise ratio is 45 dB, indicating pulse fluctuations in this condition, or that the pulse is very sensitive to environmental conditions.

[0054] Graph L in Figure 5 and Figure 6(B) illustrate the measured spectrum and autocorrelation trace of the low-order multi-bounce pulse operation regime provided by the figure-eight fiber laser at the pump power level associated with Graph L in Figure 5. The pulse structure can be inferred from the autocorrelation trace, which reveals the presence of two closely spaced pulse-bound states (pulse separation to pulse width ratio of 10) spaced 30 ps apart. Other pulse separation to pulse width ratios (e.g., 1.5 and 5) were also observed at this pump level during different self-starting events. The overlap of these bound states during cavity circulation modulates the spectrum as shown in Figure 5, with a shape dependent on the phase difference between the pulses. The RF spectrum shown in Figure 7(B) demonstrates that the signal-to-noise ratio increases by up to 55 dB in the low-order multi-bounce pulse operation regime.

[0055] Graph S in Figure 5 and Figure 6(C) illustrate the measured spectrum and autocorrelation trace of the single-pulse operation regime provided by the figure-eight fiber laser while operating only the pump 25 of the nonlinear amplification loop. The inset in Figure 6(C) shows that the autocorrelation trace is well fitted by a Gaussian profile with a 2.4 ps pulse duration. This pulse regime is stable and reproducible. The RF spectrum shown in Figure 7(C) indicates that the signal-to-noise ratio increases by up to 70 dB in the single-pulse operation regime.

[0056] For the embodiment of FIG. 3, the graph in FIG. 8 shows the measured percentage change in figure-8 fiber laser output power (squares), output pulse spectral bandwidth (circles), and output pulse temporal duration (triangles) while varying the power of the pump source 25 of the optical amplifier in the nonlinear amplification loop during operation in the single-pulse regime. The graph shows that the laser's output power is more affected by changes in pump source 25 power than the spectral bandwidth and pulse duration. Therefore, actively stabilizing the laser's output power (i.e., implementing an automatic power control loop) should lead to simultaneous control of figure-8 fiber laser pulse parameters that may otherwise vary with temperature. Controlling the output power may be achieved by forming a closed feedback loop in which the output power (as measured by detector 28 with reference to FIG. 1) is monitored and the pump source driver current (for amplifier 9, while amplifier 4 is essentially turned off) is electronically controlled (by controller 29) based on the change in output power. 8 is a characteristic of lasers that does not change with temperature, it will be appreciated that the controller 29 can have a pulse characteristic selection input and can control the output power to match the corresponding output power to the pulse characteristic selected by the controller 29. The controller can include a processor and a memory for storing the relationship between pulse characteristics and output power.

[0057] Figure 9 illustrates a schematic example of an all-normal dispersion mode-locked figure-eight fiber laser that provides stable pulse operation. The fiber laser, along with the use of a control method, operates using a single amplifier in a stable single-pulse regime. The fiber laser is shown to include a nonlinear amplification loop 31 and a unidirectional loop 32. A 2x2 central coupler 13 is included and utilized to create a path between loops 31 and 32.

[0058] The nonlinear amplification loop 31 has the same structure as the nonlinear amplification loop 11 in Figure 3. The unidirectional loop 32 is shown containing a section of rare-earth-doped gain fiber 14, with the optical pump beam 16 introduced into the gain fiber 14 via a WDM 15. An optical fiber 33 with the same dispersion characteristics as the laser cavity fiber is placed at the output of the gain fiber 14. An optical isolator 34, which ensures unidirectional operation and blocks counter-propagating pulses, is placed at the output of the fiber 33. An output coupler 35 couples out a portion of the optical power circulating within the laser cavity. A portion of the output power is photodetected and used to form the feedback loop necessary to control the laser's output pulse parameters. The power can alternatively be measured at a location separate from the output. A band-pass filter 36 is placed after the output coupler to reshape the pulses propagating within the cavity and determine the laser's center wavelength. The band-pass filter 36 can also be removed from the cavity, and the laser can continue to operate.

[0059] In one example of an all-normal, all-PM8 fiber laser, the laser 30 is an ytterbium-doped fiber oscillator delivering pulses at a wavelength of approximately 1030 nm ±1 nm. The laser delivers output pulses with a FWHM duration of approximately 15 ps at a repetition rate of approximately 8 MHz. The pulses have a FWHM spectral bandwidth of approximately 3.4 nm at a signal-to-noise ratio of 50 dB. Preferably, the laser is configured to achieve a repetition rate greater than approximately 5 MHz (more preferably greater than 10 MHz), a FWHM spectral bandwidth greater than approximately 3 nm, and pulse durations greater than approximately 10 ps at an average power greater than 1 mW. The CR 13 has a 60 / 40 coupling ratio. As those skilled in the art will recognize, this coupling ratio can be varied from the exemplary 60 / 40 ratio, but 60 / 40 is more efficient than 50 / 50 or 80 / 20. The length of the gain fiber 14 can be 0.5 m. The isolator 34 can be a low-loss isolator with a maximum insertion loss of 1 dB. This isolator ensures unidirectional propagation within the loop 32. The bandwidth of the BPF can be 11 nm. The output coupler 35 can have an 80 / 20 coupling ratio, i.e., 20% of the light is coupled out of the cavity, while 80% continues to propagate within the cavity. It has been found that the laser can operate in a stable single-pulse mode with a single amplifier (i.e., without the pumped gain fiber 14) only if the output coupler 35 manages to leave more than 80% of the light within the cavity. However, if more than 20% is coupled out of the cavity, the laser can operate with two amplifiers. The length of the optical fiber 33 can be 13 m. The length of the fiber 26 can be 1.5 m, and the length of the gain fiber 23 can be 0.5 m.

[0060] For this example, it was found that starting the first pump source 25 at a pump power in the range of approximately 400–530 mW and then starting the pump source 16 at a pump power in the range of approximately 80–140 mW self-started the figure-eight laser cavity in a multipulse operating regime. In the second stage of this sequence, the pump source 16 was maintained at the same power level as in the previous stage, and the pump source 25 was reduced to a power range of 350–450 mW. In the third stage, the pump source 16 was turned off, and the pump source 25 was maintained at the same power level as in the previous stage. In this stage, the laser switched to a single-pulse operating regime. Having the pump source 25 at a power level lower than the 350–450 mW range in this stage resulted in a loss of laser mode-locking, and amplified optical noise was emitted at the output of the laser instead of optical pulses.

[0061] It has been found that using an all-normal dispersion figure-eight fiber laser—compared to a dispersion-managed figure-eight fiber laser—it is more challenging to obtain relatively short picosecond optical pulses (less than 15 ps) with a high degree of coherence. In many cases, the pump power is adjusted so that the measured autocorrelation trace shows a broad background plus a narrow spike (called a coherent artifact) that is much shorter than the actual pulse. This case is observed when the net dispersion of the cavity is −0.16 ps. 2 to +0.15ps 2 This is never observed in a dispersion-managed figure-eight fiber laser as long as the net dispersion is within the previously specified range. 2 If , the same autocorrelation trace may be observed as far as the all-normal dispersion fiber laser case is concerned.

Claims

1. A mode-locked, figure-eight fiber laser, comprising: a fused fiber coupler for connecting two fiber loops; a bidirectional loop connected to the coupler, the bidirectional loop having a gain element and a first pump source; a unidirectional loop connected to the coupler, the unidirectional loop including a gain element, a second pump source, an isolator, a filter, and an output coupler; Equipped with the laser is configured to initiate high-order multi-pulse state operation when both the first pump source and the second pump source are driven at high power; the laser is configured to transition from the higher-order multipulse state operation to the lower-order multipulse state operation when the first pump source is driven at a lower power; the laser is configured to transition from the low-order multi-pulse state of operation to perform a stable single-pulse state of operation when the second pump source is driven at zero power; Mode-locked, figure-eight fiber laser.

2. 10. The laser of claim 1 further comprising a controller connected to the first pump source and the second pump source and configured to drive both the first pump source and the second pump source at a high power to initiate a high-order multipulse state of operation, then drive the first pump source at a lower power for a transition to a low-order multipulse state of operation, and then drive the second pump source at zero power for a transition to the stable single-pulse state of operation.

3. 3. The laser of claim 2, wherein the controller is configured to vary the power levels of the first and second pump sources in predetermined increments with a wait time between successive increments as power levels change from the high power for initiating a high-order multipulse state operation, then to drive the first pump source at a lower power for a transition to a low-order multipulse state operation, and then to drive the second pump source at zero power for a transition to a stable single-pulse state of operation.

4. 3. The laser of claim 2, wherein once the laser has transitioned to a stable single-pulse state of operation, the controller is configured to drive the second pump source at a constant low level of power to produce a wider range of output powers, pulse durations, and spectral bandwidths from the laser.

5. 5. The laser of claim 4 wherein said controller is configured with a pulse characteristic selection input to transition said pulse characteristic selection input to a desired output power level.

6. 6. The laser of claim 1, wherein the unidirectional loop and the bidirectional loop comprise polarization-maintaining fibers and components.

7. 7. The laser of claim 1, wherein the laser includes a length of optical fiber having group velocity dispersion of opposite sign to other fibers in the laser for dispersion management, and wherein the single-pulse condition of operation, in use, provides a stretched pulse.

8. 8. The laser of claim 1, wherein the isolator, the filter and the output coupler comprise an optical circulator and a chirped fiber Bragg grating.

9. 9. The laser of claim 1, wherein the gain element of the bidirectional loop comprises a section of rare-earth doped fiber.

10. 10. The laser of claim 1, wherein the gain element of the unidirectional loop comprises a section of rare-earth doped fiber.

11. 11. The laser of claim 9 or 10, wherein the section comprises an ytterbium-doped fiber.

12. Net cavity group velocity dispersion is -0.16 ps ２ +0.15 ps ２ 12. The laser of claim 1, wherein the wavelength is in the range of

13. 13. The laser of any one of claims 1 to 12, wherein the filter (or CFBG) has a spectral bandwidth of at least 7 nm.

14. the laser in the single-pulse state of operation; Full width at half maximum (FWHM) duration less than 15 ps; a repetition rate greater than 5 MHz; a FWHM spectral bandwidth greater than about 1 nm, and Average output power above 1mW 14. The laser of claim 1 configured to generate output pulses having at least one of the group consisting of:

15. 1. A method for generating a stable train of laser pulses, comprising: i) a fused fiber coupler for connecting two fiber loops; a bidirectional loop connected to the coupler, the bidirectional loop having a gain element and a first pump source; a unidirectional loop connected to the coupler, the unidirectional loop including a gain element, a second pump source, an isolator, a filter, and an output coupler; wherein the laser is configured to drive both the first pump source and the second pump source at high power to initiate a high-order multipulse state of operation, drive the first pump source at low power for transition to a low-order multipulse state of operation, and drive the second pump source at zero power for transition to a stable single-pulse state of operation; ii) driving both the first pump source and the second pump source at high power to initiate high-order multi-pulse state operation; iii) driving the first pump source at a lower power to transition to a lower-order multi-pulse state of operation; iv) driving the second pump source at zero power to transition to a stable single-pulse state of operation; A method comprising:

16. 16. The method of claim 15, wherein the figure-eight laser comprises a laser according to any one of claims 1 to 14.

17. A method as described in claim 15 or 16, further comprising driving at least one of the first pump source and the second pump source for at least one intermediate order multi-pulse state operation or for the single pulse state of operation.

18. A method described in any one of claims 15 to 17, further comprising increasing the power of the first pump source during the single pulse state of operation to increase the power of the pulse.

19. The method of claim 15, further comprising using a length of optical fiber in the laser to create a stretched pulse during the single-pulse state of operation.

20. A method described in any one of claims 15 to 19, further comprising controlling the net dispersion of the cavity and filtering light of specific wavelengths using a chirped fiber Bragg grating.

21. The method of any one of claims 15 to 20, further comprising introducing gain into the optical signal propagating through the rare-earth doped gain fiber.

22. The method of claim 15, wherein driving both the first pump source and the second pump source at a high power to initiate higher order multipulse state operation includes varying the power levels of the first pump source and the second pump source in predetermined increments with a wait time between successive increments as power levels change from the high power to initiate higher order multipulse state operation.

23. The method of claim 15, further comprising driving the second pump source at a constant low level of power to produce a wider range of output power, pulse duration, and spectral bandwidth from the laser once the laser has transitioned to a stable single-pulse state of operation.

24. The method of claim 23, further comprising receiving, at a controller, a pulse characteristic selection input and transitioning the pulse characteristic selection input to a desired output power level.

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