Frequency-multiplicable swept source with large scanning range

Abstract

A frequency-multiplicable swept source with a large scanning range, the swept source comprising: a ring cavity for generating fundamental-frequency swept laser, an optical frequency-multiplying module for multiplying a scanning rate, and a boosting-type semiconductor optical amplification module used for enhancing an output power. The swept source enables, by means of driving a rapid-scan filter, the ring cavity to generate swept laser having a sweep rate of 41 kHz and a sweep range of 97 nm; and by means of an optical frequency multiplier, the sweep rate of the optical pulse triples to reach 124 kHz, and the output power thereof reaches 20 mw or above. The swept source also enables the sweep range to reach the maximum FSR of the rapid-scan filter while using an optical frequency-multiplying means, thereby overcoming the problem of the sweep range not reaching the FSR when light-source duty cycle modulation is used for performing frequency multiplying on the swept source; in addition, the problem of the sweep range being lower than the maximum FSR where the rapid-scan filter is not at a resonant frequency can also be solved, thereby satisfying the performance requirements of an OCT system in the current market. Moreover, the structure of the swept source is mostly made of optical fibers and is thus easily integrated as a whole.

Description

A frequency-swept light source with doubled frequency and large scanning range Technical Field

[0001] The invention belongs to the fields of laser technology, spectrum measurement, biomedicine, etc., and particularly relates to a frequency-sweeping light source capable of frequency doubling and having a large scanning range. Background Art

[0002] A swept-frequency laser is a laser capable of continuously tuning its output frequency. Unlike traditional fixed-frequency lasers, a swept-frequency laser can vary its output frequency within a frequency range. This characteristic makes swept-frequency lasers widely used in fields such as biomedicine, spectroscopy, lidar, and optical communications.

[0003] The operating principle of a swept-frequency laser is based on changing key parameters of the laser resonator, achieving continuous frequency variation through adjustment of optical components. This makes swept-frequency lasers an important tool in scientific research, industrial inspection, and medical imaging, providing strong support for precise optical measurements and high-resolution spectral analysis.

[0004] The successful application of swept-frequency lasers in diverse fields demonstrates their unique advantages. In optical measurement, they provide highly precise frequency tuning, enabling the detection of subtle optical features in spectral analysis. Furthermore, their continued innovation and development in fields such as optical communications and medical imaging provide a powerful impetus for advancing cutting-edge research and practical applications of laser technology.

[0005] One of the most notable applications of swept-frequency lasers is their application in swept-frequency optical coherence tomography (OCT). OCT is a high-resolution optical imaging technique that can non-invasively acquire microscopic images of tissue structures. The frequency tuning capability of swept-frequency lasers enables OCT systems to achieve higher depth resolution, thereby improving the accuracy of imaging biological tissue structures. In ophthalmology, OCT has become an important tool for diagnosing and monitoring retinal diseases.

[0006] In existing technologies, frequency-swept lasers fail to achieve the filter's free spectral range (FSR) due to a mismatch in the resonant frequency of the piezoelectric ceramics in their rapidly tunable filters. This limits their ability to cover a wider frequency sweep range in applications such as OCT. Furthermore, precise matching of the cavity length and resonant frequency in Fourier domain mode locking (FDML) frequency-swept cavities is difficult, and the frequency sweep rate issue also restricts their performance in high-speed, real-time applications such as OCT. These limitations can impact applications requiring rapid response and precise measurement. Summary of the Invention

[0007] To overcome the technical drawbacks of swept-frequency light sources that fall short of the FSR when not at resonant frequency, or the reduced sweep range caused by frequency-doubling duty cycle modulation, the present invention proposes a swept-frequency light source with a wide sweep range and frequency-doubling capability. The light source comprises a ring cavity for generating a fundamental frequency swept laser, an optical frequency multiplier for doubling the sweep rate, and a booster-type semiconductor optical amplifier for increasing output power. First, the ring cavity generating the fundamental frequency swept laser generates a low-frequency swept laser with a 33% duty cycle. The optical frequency multiplier then splits a portion of the low-frequency swept laser and delays it through an 821m-long single-mode optical fiber. This delay is offset from the low-frequency swept laser, achieving a tripling of the frequency.

[0008] The fast-sweep filter uses a sine wave drive. If the semiconductor optical amplifier is modulated with a low duty cycle, the sweep range will inevitably decrease compared to the unmodulated state. However, by overdriving the fast-sweep filter, the sweep range can be maintained near the maximum value even when the semiconductor optical amplifier is modulated with a low duty cycle. Overdriving the fast-sweep filter beyond the FSR causes the laser to pass through multiple transmission steps of the filter. This overcomes the technical drawbacks of swept light sources that do not reach the FSR when the sweep range is not at the resonant frequency, or the reduced sweep range caused by using double-frequency duty cycle modulation.

[0009] The resulting swept laser is then output through a booster-type semiconductor optical amplifier to boost output power. This method ultimately achieved a swept laser output with a sweep range of 97nm, a sweep rate of 124kHz, and an output power of 20mW.

[0010] In order to achieve the above object, as shown in FIG1 , the present invention is implemented by the following technical solutions:

[0011] The present invention is a swept-frequency light source with a large scanning range and capable of frequency doubling. The swept-frequency light source comprises a ring cavity for generating a fundamental frequency swept-frequency laser, an optical frequency doubling module for doubling the scanning rate, and a booster-type semiconductor optical amplifier module for increasing the output power.

[0012] The ring cavity includes a booster semiconductor optical amplifier, two optical isolators, two polarization controllers, a 50:50 optical coupler, a 4.9km section of single-mode optical fiber, a fast scanning filter, and a driving circuit for the fast scanning filter.

[0013] The spontaneous radiation light generated by the semiconductor optical amplifier in the ring cavity enters the fast scanning filter, which filters out a narrowband light. The fast scanning filter is driven by a 41kHz sine wave to change the center wavelength of the filter, thereby achieving the effect of frequency sweeping. By overdriving the fast scanning filter beyond the filter's FSR, the laser passes through multiple transmission orders of the filter, allowing the light to still produce a wide range of frequency-sweep lasers under 33% duty cycle modulation.

[0014] The cavity length of the ring cavity is the same as the modulation period of the fast scanning filter. Therefore, when the filtered narrowband light reaches the fast scanning filter after circling one circle, the filter's filtering center wavelength is exactly the same as the wavelength of the transmitted light. That is, all wavelengths in the cavity will not be lost because the light does not correspond to the filter's center wavelength when entering the filter. During this process, the semiconductor optical amplifier is modulated by a square wave with a 33% duty cycle. At the same time, the gain of the semiconductor optical amplifier can be adjusted by the polarization controller in the ring cavity. The generated swept laser is an optical pulse with a period of 41kHz and a duty cycle of 33% in the time domain.

[0015] Since the laser generated in the ring cavity has a 66% vacancy in the time domain, which is exactly twice the 33%, after the light pulse enters the optical frequency doubling module, the doubled light pulse should be roughly the same in power, that is, a light pulse with a frequency of 124kHz and a duty cycle of about 100% is generated; as a preferred embodiment of the invention, after the light pulse enters the optical frequency doubling module, the light pulse is first separated by a 30:70 optical coupler to obtain 30% of the light, and the light passes through the optical attenuator and is reflected at the 90° first Faraday rotator. The reverse transmission light and the forward transmission light are exactly orthogonally polarized, thereby avoiding mutual influence and nonlinear effects. The remaining 70% of the light passes through an 821m single-mode optical fiber and is then The 47:53 optical coupler splits out 47% of the light, which is reflected by the 90° second Faraday rotator. The reflected light passes through 821m of single-mode fiber again, and is exactly 8us away from the first beam of reflected light in the time domain. The remaining 53% of the light passes through another 821m of single-mode fiber and is reflected by the 90° third Faraday rotator. In the time domain, it is exactly 16us away from the first beam of reflected light. After passing through an optical attenuator and calculating the splitting ratio and the loss of each light passing through the optical fiber length, the doubled light pulses have roughly the same power. The optical frequency doubler triples the sweep rate of the light pulse, and the output power reaches more than 20mW, generating a light pulse with a frequency of 124kHz and a duty cycle of approximately 100%.

[0016] Finally, the power of the frequency-doubled optical pulse is increased by a booster-type semiconductor optical amplifier. Taking into account the effects of backward spontaneous radiation and gain unevenness of the third semiconductor optical amplifier, a third optical isolator and a third polarization controller are installed at the input port of the third semiconductor optical amplifier to reduce the nonlinear effects generated during amplification.

[0017] The beneficial effects of the present invention are:

[0018] The ring cavity length of the present invention matches the period of the sinusoidal wave driving the fast scanning filter, allowing lasers of multiple wavelengths to oscillate simultaneously within the cavity and in a temporal sequence. The light source overdrives the fast scanning filter with a high voltage to exceed the FSR, allowing the laser to pass through multiple transmission steps of the filter. This allows the light to generate a wide range of swept-frequency lasers even under low duty cycle modulation when using an optical frequency doubler.

[0019] The light source uses an optical frequency doubler to match the loss and splitting ratio of the single-mode optical fiber, and uses an optical attenuator for fine-tuning, so that the frequency of the swept laser is tripled and the power of the three beams is the same. This light source is of great significance for achieving a wider range and higher frequency for swept lasers whose sweep range is reduced due to optical frequency doubling or whose driving frequency is not at the resonant frequency of the piezoelectric ceramic.

[0020] The present invention increases the voltage of the fast scanning filter through an overdriving method, thereby avoiding the situation in which the spectral range is reduced due to duty cycle modulation of the optical amplifier when using an optical frequency doubling module, and the scanning rate of the fast filter in the FDML scheme and the piezoelectric ceramic are mismatched when the cavity length is limited. The output spectral range is only 57nm when the overdriving method is not used, while the output spectral range is 97nm when the overdriving method is used, that is, the output spectral range is increased by about 40nm through this method. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG1 is a structural diagram of a frequency-sweeping light source capable of frequency doubling and a large scanning range according to the present invention.

[0022] FIG2 is a spontaneous emission spectrum diagram of the semiconductor optical amplifier of the present invention.

[0023] FIG. 3 is a spectrum diagram generated when overdriving and duty cycle modulation are not adopted in the present invention.

[0024] FIG4 is a spectrum diagram of the present invention without overdriving and using duty cycle modulation.

[0025] FIG5 is a spectrum diagram of the present invention using overdrive and 33% duty cycle modulation.

[0026] FIG6 is a time domain diagram of the present invention when overdrive is adopted and 33% duty cycle modulation is not frequency multiplied.

[0027] FIG7 is a spectrum diagram of the frequency-doubled device after amplification of the present invention.

[0028] FIG8 is a time domain diagram of the frequency-multiplied circuit of the present invention after amplification. DETAILED DESCRIPTION

[0029] The following diagrams illustrate embodiments of the present invention. For clarity, many practical details are included in the following description. However, it should be understood that these practical details are not intended to limit the present invention. In other words, in some embodiments of the present invention, these practical details are not essential.

[0030] Example 1

[0031] The present invention discloses a frequency-doubling swept-frequency light source with a large scanning range, which is used in a ring cavity for generating a fundamental frequency swept-frequency laser. The output end of a first semiconductor optical amplifier is connected to the input end of a first optical isolator, which is connected to the input port of a first polarization controller. The output port of the first polarization controller is connected to a 4.9 km section of single-mode optical fiber. The other end of the 4.9 km section of single-mode optical fiber is connected to the input end of a fast scanning filter, which is connected to the input end of a 50:50 optical coupler. One of the 50-degree splitting optical ports serves as a port for outputting the fundamental frequency swept-frequency laser, and the other 50-degree splitting optical port is connected to the input of a second polarization controller. The output port of the second polarization controller is connected to the input port of a second optical isolator, which is connected to the input port of the first semiconductor optical amplifier.

[0032] In this embodiment, only a ring cavity that generates a fundamental frequency swept laser is used for the experiment. The first semiconductor optical amplifier uses the BOA1132S from Thorlabs. The DC bias current of the semiconductor optical amplifier is set to 650mA. At the same time, the signal generator generates a 41kHz sine wave with a DC bias of 10.5V and a peak value of 12V. The sine wave is loaded onto a fast scanning filter, thereby generating a 41kHz swept laser.

[0033] The working principle of the present invention is as follows: spontaneous radiation generated by a semiconductor optical amplifier in the cavity is filtered through a rapid scanning filter to produce a single-wavelength beam. After circling once in the cavity, it passes through the optical amplifier again for gain. By driving the rapid scanning filter with a sinusoidal voltage, the center wavelength of the rapid scanning filter can be changed, thereby generating a wide range of swept-frequency light. The time it takes for the light to circulate once in the cavity matches the signal period loaded on the filter, making the swept-frequency laser output spectrum flatter. The frequency of the light in the resonant cavity needs to accurately match the frequency of the filter, and the following formula must be satisfied:

[0034] Where L is the length of the entire resonant cavity, c is the speed of light in the optical fiber, n is the number of times the light travels back and forth in the resonant cavity, and f is the scanning frequency of the tuning filter.

[0035] FIG2 is a spontaneous emission spectrum of the first semiconductor optical amplifier. It can be seen from the figure that the main power of the spontaneous emission is concentrated in the range of 1218 nm to 1363 nm.

[0036] Figure 3 shows the spectrum when duty cycle and overdrive modulation are not used. It can be seen that the spectrum is relatively flat, but its scanning range is reduced because the frequency of the driving voltage is not at the resonant frequency of the piezoelectric ceramic.

[0037] Example 2

[0038] In this embodiment, a 33% duty cycle modulation is applied to the first semiconductor optical amplifier within the ring cavity. A signal generator generates a 41 kHz square wave with a high level of 1.35V and a low level of 0V. The DC bias of the first semiconductor optical amplifier is 0 mA, and the voltage-to-current conversion coefficient is 500 mA / V. This means that the current input to the first semiconductor optical amplifier is 675 mA at a high level and 0 mA at a low level. The phase of the square wave is adjusted so that the center of the square wave occupies the intersection of the positive and negative half-cycles of the sine wave driving the rapid scanning filter. Modulating the first semiconductor optical amplifier using this method reduces the frequency sweep range. The principle is that the square wave occupies 16.5% of the sine wave cycle on either side of the intersection, corresponding to a phase difference of 33 / 100π on either side of the sine wave. This means that under square wave modulation, the sine wave of the rapid scanning filter only has an effective sweep range of 86.07%, a reduction of approximately 14% in the maximum sweep range compared to the case without duty cycle modulation.

[0039] FIG4 is a spectrum diagram when the duty cycle is used but the overdrive mode is not used for modulation. It can be seen that the scanning spectrum range is reduced by about 15 nm compared with that in Example 1.

[0040] Example 3

[0041] Based on Example 2, this embodiment adopts an overdrive method to drive the fast scanning filter in the ring cavity. The signal generator generates a 41kHz sine wave with a DC bias of 4.7V and a peak-to-peak value of 6.4V. The phase of the square wave driving the semiconductor optical amplifier is adjusted so that the high-level phase is in the rapidly changing part of the sine wave. The sine wave is doubled by the power amplifier and then loaded onto the fast scanning filter. This operation drives the fast scanning filter to exceed the FSR of the filter, allowing the laser to pass through multiple transmission orders of the filter.

[0042] Figure 5 shows the spectrum when both duty cycle and overdrive modulation are used. It can be seen that the scanning spectrum range is increased by about 40nm compared to when only duty cycle modulation is used, and the entire scanning range is about 97nm.

[0043] FIG6 is a time domain diagram when the duty cycle and overdrive modulation modes are used simultaneously. It can be seen that each swept laser pulse appears at a frequency of about 41 kHz and the duty cycle is about 33%.

[0044] Example 4

[0045] This embodiment adds an optical frequency multiplier to double the scan rate and a booster-type semiconductor optical amplifier to increase power, based on Example 3. The semiconductor optical amplifier uses a Thorlabs BOA1130S, and drives the laser ring cavity using the method of Example 3. The optical power reflected at the first Faraday rotator is 30% of the total optical power entering the optical frequency multiplication module. The single-mode fiber model is SMF-28e+, and its loss in the 1310nm band is 0.33dB / km. Therefore, the optical power reflected at the second Faraday rotator should be 28.262% of the total optical power entering the optical frequency multiplication module, and the optical power reflected at the third Faraday rotator should be 27.38%. The optical attenuator before the first Faraday rotator adjusts the loss to ensure that the three optical pulses have approximately the same power.

[0046] FIG7 is the final spectrum after frequency doubling and power amplification, and the scanning range is about 97 nm.

[0047] FIG8 is a time domain diagram after frequency multiplication and power amplification. In the time domain, the pulse frequency is about 124 kHz and the duty cycle is about 100%.

[0048] The foregoing is merely an embodiment of the present invention and is not intended to limit the present invention. It will be apparent to those skilled in the art that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are intended to be included within the scope of the claims of the present invention.

Claims

1. A frequency-swept light source capable of frequency doubling and a large scanning range, characterized in that: The swept-frequency light source capable of frequency doubling and a large scanning range comprises a ring cavity for generating a fundamental frequency swept-frequency laser, an optical frequency doubling module for doubling the scanning rate, and a booster-type semiconductor optical amplifier module for increasing the output power; The ring cavity comprises a first semiconductor optical amplifier, a first optical isolator, a first polarization controller, a 4.9 km single-mode optical fiber, a fast scanning filter, a 50:50 optical coupler, a second polarization controller, and a second optical isolator, which are sequentially connected to form a ring according to the transmission direction of the optical signal. The output end of the 50:50 optical coupler is two 50-degree optical ports, one of which serves as the port for the ring cavity to output the fundamental frequency swept laser, and the other 50-degree optical port is connected to the input of the second polarization controller. The fast scanning filter is driven by a 41 kHz sine wave and overdriven to exceed the filter's FSR. The cavity length of the ring cavity is the same as the modulation period of the fast scanning filter. The first semiconductor optical amplifier is modulated by a square wave with a 33% duty cycle, and the gain of the first semiconductor optical amplifier is adjusted by the first polarization controller and the second polarization controller to generate optical pulses with a time domain period of 41 kHz and a duty cycle of 33%. The optical pulse achieves a frequency sweep rate tripled to 124 kHz through an optical frequency multiplier, and the output power reaches more than 20 mW; the optical frequency multiplication module includes an optical circulator, which includes a first port, a second port, and a third port; the booster-type semiconductor optical amplifier module is connected in sequence and includes: a third polarization controller, a third optical isolator, and a second semiconductor optical amplifier according to the direction of optical signal transmission; the port of the 50:50 optical coupler that outputs the fundamental frequency sweep laser of the ring cavity is connected to the first port of the optical circulator, and the third port of the optical circulator is connected to the third polarization controller.

2. The swept frequency light source with doubled frequency and large scanning range according to claim 1, characterized in that: The second port of the optical circulator is connected to the input port of a 30:70 optical coupler, wherein the 30 splitting port is connected to an optical attenuator, the optical attenuator is connected to a first Faraday rotator, and the 70 splitting port is connected to a 821m first single-mode optical fiber; the other end of the first single-mode optical fiber is connected to the input end of a 47:53 optical coupler, wherein the 47 splitting port is connected to a second Faraday rotator, and the 53 splitting port is connected to another 821m second single-mode optical fiber; and the other end of the second single-mode optical fiber is connected to a third Faraday rotator.

3. The swept frequency light source with doubled frequency and large scanning range according to claim 1, characterized in that: The semiconductor optical amplifier has a central wavelength of 1300 nm, a 3dB bandwidth of 87 nm, and a small signal gain of 30 dB.

4. The swept frequency light source with doubled frequency and large scanning range according to claim 1, characterized in that: The fast scan filter has an FSR of 97 nm, a resonant frequency of 28 kHz, and a finesse of 980.

5. The swept frequency light source with doubled frequency and large scanning range according to claim 4, characterized in that: The swept frequency laser output by the swept frequency light source has a sweep frequency range of 97 nm, a sweep rate of 124 kHz, and an output power of 20 mW.

6. The swept frequency light source with doubled frequency and large scanning range according to claim 1, characterized in that: The output of the third polarization controller is connected to the input port of the third optical isolator, the output port of the third optical isolator is connected to the input port of the second semiconductor optical amplifier, and the output port of the second semiconductor optical amplifier is the output port of the swept frequency light source with a large scanning range and capable of doubling frequency.

7. The swept frequency light source with doubled frequency and large scanning range according to claim 6, characterized in that: The booster-type semiconductor optical amplifier module for increasing output power has a central wavelength of 1285 nm, a 3 dB bandwidth of 87 nm, and a small signal gain of 30 dB.

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