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Abstract
We demonstrate for the first time a strain-controlled all polarization-maintaining (PM) fiber Lyot filter based on a piezoelectric lead zirconate titanate (PZT) fiber stretcher. This filter is implemented in an all-PM mode-locked fiber laser to serve as a novel wavelength-tuning mechanism for fast wavelength sweeping. The center wavelength of the output laser can be tuned across a range from 1540 nm to 1567 nm linearly. And the strain sensitivity achieved in the proposed all-PM fiber Lyot filter is 0.052nm/με, which is 43 times higher than that achievable by other strain-controlled filters such as a fiber Bragg grating filter (0.0012nm/με). Wavelength-swept rates up to 500 Hz and wavelength tuning speeds up to 13,000 nm/s are demonstrated, which is hundreds of times faster than what is attainable with conventional sub-picosecond mode-locked lasers based on mechanical tuning methods. This highly repeatable and swift wavelength-tunable all-PM fiber mode-locked laser is a promising source for applications requiring fast wavelength tunability, such as coherent Raman microscopy.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, there are growing research interests in wavelength tunable mode-locked lasers for applications such as spectroscopy [1], optical communications [2], fiber-optic sensing [3], and biomedical imaging [4]. Especially in biological imaging research, there is great interest in stimulated Raman scattering (SRS) microscopy with wavelength tunable laser source to obtain hyperspectral images. Compared with conventional microscopy approaches for instance, multiphoton fluorescence microscopy, SRS microscopy has many advantages, such as label-free detection and small molecule detection capability [5]. To capture various kinds of matter with these advantages while realizing real-time imaging, the wavelength tunable laser source for SRS microscopy needs to satisfy several requirements: (i) picosecond pulse for efficient signal-to-noise ratio, (ii) wavelength tuning at near-infrared regions around 0.8-1.5 $\mathrm {\mu }$m, (iii) stable and repeatable operation out of the laboratory environment, (iv) compactness for practical usage, and most importantly, (v) fast wavelength-swept rate [6]. Until now, wavelength-tunable pulse lasers have been achieved using a few approaches. The most common methods involve using different variations of intracavity filters. [7] For instance, an axial-applied strain-controlled fiber Bragg grating (FBG) has been used as a wavelength tuning mechanism [8]. However, the laser only delivers 18.9 ps long pulses over a limited wavelength range of merely 8.6 nm. A phase-shifted long-period fiber grating (LPFG) was demonstrated to realize a wavelength-tunable fiber laser [9]. By heating the grating from 30$^\text {o}$C to 80$^\text {o}$C, the wavelength can be tuned over 20 nm. However, the thermally-controlled process makes the tuning speed extremely slow. Another way to utilize LPFG is bending control [10]. Through bending the LPFG, the central wavelength can be tuned continuously from 1582.02 nm to 1597.29 nm. The bending approach achieves a fast wavelength tuning speed than the thermally-controlled method; however, it cannot achieve an as wide tuning range due to the mechanical damage threshold of the LPFG. Recently, another demonstration using tunable fiber grating was reported [11]. The wavelength tuning is implemented with a fiber birefringence filter consisting of a piece of PM fiber, a fiber Brewster grating and two polarization controllers (PC). However, the two PCs in this design must be carefully adjusted to get the maximum tuning range. This feature makes such a design not suitable for practical usage. In addition to the aforementioned approaches, all-fiber construction cavity has also been widely studied for its compactness. For example, a wavelength-tuning passively mode-locked all-fiber laser based on cascaded dual single mode fiber—graded index multimode fiber—single mode fiber (CD-SMS) structure has been reported [12]. The wavelength was tuned in the range of 1533 nm to 1573 nm by tuning the angles of two PC paddles. However, the wavelength cannot be continuously tuned, and only 5 peaks were achieved in the tuning range. Another approach with all fiber structures is using a Sagnac interferometer-based fiber optical loop mirror (FOLM) as a wide wavelength-tunable filter [13]. By controlling the wavelength reflection of FOLM thermally, a tunable mode-locking was achieved in the range of 1543.2 nm to 1569.5 nm. However, the tuning speed are limited by the thermal adjustment process.
The aforementioned approaches are all realized in non-PM laser cavity with mechanically-, manually-, or thermally-controlled mechanisms. Thus, they all have disadvantages such as slow wavelength tuning speed and instability, making those methods unsuitable for practical usage. Previously, we have reported a Polarization-maintaining all-fiber wavelength-tunable mode-locked laser based on a thermally controlled Lyot filter [14]. By tuning the temperature of the all fiber Lyot filter section, a femto/picosecond mode-locking with a wavelength tuning range of 1546 nm to 1571 nm was achieved. Since the whole system was all PM fiber structure, such design is highly stable and meets the above requirements (i)–(iv). However, because it utilized a slow temperature control mechanism, the wavelength tuning speed problem remains. In order to solve that, an improvement in the control mechanism is demanded. To achieve fast wavelength tuning, an intracavity filter incorporating a galvanometer mirror and a diffraction grating was employed in a mode-locked fiber laser [6]. By controlling the filter, a 30 nm wavelength tunability with 6,000 nm/s tuning speed was achieved. However, the laser cavity is bulky and complicated and can only generate rather long 9-15 picosecond pulses. Other fast wavelength tuning methods, such as Fourier domain mode-locking (FDML), can achieve megahertz-level wavelength tuning rates. [15,16] However, such lasers can only generate nanosecond-level pulses with low peak power, which results in low stimulated Raman gain signal levels.
In this paper, we demonstrate a fast wavelength-swept all-PM fiber mode-locked laser based on strain-controlled Lyot filter. A lever control relationship with an 1,100 times magnification factor between the wavelength and the applied strain is achieved with the proposed filter. This is for the first time such wavelength-tuning mechanism has been reported to the best of our knowledge. By applying 24$\mu \epsilon$ to 542$\mu \epsilon$ to the filter, the wavelength can be tuned from 1540 nm to 1567 nm. This result is approximately 43 times more efficient than the previous grating or strain-tuning FBG approaches [8,9]. It can be utilized to achieve highly repeatable hundreds of Hz-order wavelength-swept mode-locked fiber lasers, and the wavelength tuning speed is up to 13,000 nm/s in our laser cavity.
2. Principle
2.1 Fiber Lyot filter
A Lyot filter is an optical device with wavelength-dependent power transmission. It incorporates a sequence of birefringent materials and polarizers [17]. For a fiber-based Lyot-filter, the basic structure comprises a polarization controller (PC), a piece of birefringent fiber (such as PANDA fiber) and a polarizer. The filter works as follows: Firstly, the PC modifies the polarization state of the light propagating into the filter section. After The PC, the light is usually linearly polarized. However, the polarization is not along the fast or slow axis of the PANDA fiber. Thus, it splits into two polarization components: one is along the fast axis, while the other is along the slow axis. When the light propagates into the PANDA fiber, these two components will experience a walk-off because of the birefringence. The speed of the walk-off depends on the wavelength of light. Thus, the synthesized polarization states of light of different wavelengths vary at the end of the PANDA fibre. Based on that, the polarizer gives a unique loss to each wavelength component. Therefore, the whole structure is a wavelength-dependent filter.
The transmission property of a primary Lyot filter with one piece of birefringent fiber can be analyzed as follows [18]:
where $[E_{out} ]$ and $[E_{in} ]$ are the electric vector of output and input light.[M] [P] and [J] are the Jones matrices of the polarizer, the PC and the polarization-maintaining fiber (PMF), respectively. To simplify the calculation, the polarizer can be set along the x (fast) axis. Therefore, the Jones matrices can be written as [19]:According to Eq. (4), the FSR of a Lyot filter is inversely proportional to the length of the PANDA fiber and the value of birefringence.
The modulation depth of the Lyot-filter is determined by $\theta$, which can be controlled by PC. Since a PC cannot be implemented for all-PM fibre structure, an angle-spliced point can be used to generate the initial polarization mismatch. In order to achieve a maximum modulation depth, the slicing angle can be set to 45$^\text {o}$. According to our measurement, the loss caused by such angled splicing is less than 0.1 dB, which is negligible.
2.2 Wavelength tuning mechanism
According to Eq. (3), The mathematical simulation of the transmission curve of Lyot filters is shown in Fig. 1. Notice that the simulation is done in the frequency domain by replacing the $\lambda$ in Eq. (3) and Eq. (4) with C/f, where C is the speed of light in vacuum and f is the frequency of the light. We perform this replacement operation because the Lyot filter is periodic in the frequency domain. Therefore, such an operation will help us to analyze and summarize the tuning mechanism. The initial polarization mismatch angle $\theta$ is set to 45$^\text {o}$, and the length of PANDA fiber is set to 21 cm to ensure a wide tuning range according to our experience [14]. Figure 1(a) shows the frequency domain transmittance curve of a Lyot filter when the ${\Delta }n$ of standard PANDA fiber is $3.6{\times }10^{-4}$. Lyot filter is a frequency-dependent periodic filter, and the 50th order transmittance peak located at approximately 193 THz frequency point corresponds to 1550 nm wavelength. As described above in Eq. (4), the FSR (period) of the Lyot filter is inversely proportional to ${\Delta }n$ and $L_{PMF}$. Thus, Fig. 1(b) shows that by increasing the value of ${\Delta }n$, the FSR of the Lyot filter decreases. Furthermore, due to the periodic filter nature of the Lyot-filter, from low-order peaks to higher-order peaks, the difference in FSR will accumulate. Therefore, even a tiny change in the FSR will lead to significant shifts in the transmittance peak position, and the peaks of higher order move further away. Based on this property, by adjusting ${\Delta }n$, the position of the transmittance peak can be moved with a magnification factor, and at around 1550 nm, the magnification factor is 50. Thus, a wide wavelength tuning range can be achieved with minor changes in the filter parameters. Utilizing this principle, We have previously reported a thermally controlled wavelength-tunable mode-locked laser. However, due to the time-consuming heating process, fast tunability was not achieved. Therefore, an improvement to this tuning mechanism is needed.
Fig. 1. Transmittance curve of Lyot filter when the angle of initial polarization mismatch is set to 45$^\text {o}$ and the length of PANDA fiber is set to 21 cm. (a) is the frequency domain transmittance curve when the birefringence ${\Delta }n$ of PANDA fiber is $3.6{\times }10^{-4}$. It is a periodic filter and at 193 THz is the 50th order peak. (b) is the comparison between transmittance curve of ${\Delta }n = 3.6{\times }10^{-4}$ (blue line) and that of ${\Delta }n = 3.7{\times }10^{-4}$ (orange line). (b) is zoomed in and focused on 0 THz to 50 THz range for a clearer comparison.
Besides temperature, PM fiber is also sensitive to stress-induced deformation [22]. Because of that, strain sensor based on PM fiber structure has been extensively studied in fiber sensing field [23–26]. Since Poisson’s ratio of fiber cladding and the stress-applying parts (SAPs) are different, a lateral strain difference is produced when the PANDA fiber is stretched axially. Therefore, the stress distribution in the fiber cross-section will change, and the value ${\Delta }n$ of the fiber changes accordingly. The strain coefficients of birefringence of the PANDA fiber we utilize is measured to be $8{\times }10^{-9}/\mu \epsilon$ [27]. Compared with ${\Delta }n = 3.6{\times }10^{-4}$, the following conclusion can be obtained: When the PANDA fiber is stretched with a 0.0001% length change (1$\mu \epsilon$), a 0.0022% change ($8{\times }10^{-9}/3.6{\times }10^{-4}$) in ${\Delta }n$ will occur. According to Eq. (4), the change in ${\Delta }n$ and the $L_{PMF}$ give the same impact to the FSR change. Thus, it can be concluded that using strain to tweak the value of ${\Delta }n$ is a magnification control process. Based on the above analysis, the magnification factor is 22 (0.0022%/0.0001%). Therefore, the whole wavelength tuning mechanism based on strain control can be described as follow: First, the applied strain will change the value of ${\Delta }n$ with a magnification factor of 22. Then, the change in ${\Delta }n$ will lead to an FSR change (The length change/strain itself can also affect FSR; however, it is too small to be noticed). Finally, The change in FSR will shift the position of transmittance peaks with a magnification factor of 50 around 1550 nm. Ultimately, the wavelength of the mode-locking output is tuned. Overall, by using a strain-controlled all-PM fiber Lyot filter, a two-stage control mechanism with a total magnification factor of 1,100 is achieved. Such a mechanism can be implemented for fast and broadband wavelength sweeping.
3. Experiment and results
3.1 Laser setup
The proposed wavelength-tunable mode-locked laser schematic is shown in Fig. 2. To simplify the cavity structure, a multifunctional fast-axis-blocked isolating wavelength-division coupler with an additional tapping port (PM-TIWDM) is used. It serves as an isolator, a WDM coupler, a polarizer, and a 50% output coupler. A 980 nm pump laser diode (LD), a 1 m PM-EDF (Nufern PM-ESF-7/125, 15ps/nm/km) and a 2.5 m PM fiber (Fujikura SM15-PS-U25A, 15ps/nm/km) constitute a ring cavity. A carbon nanotube (CNT) sprayed connector, which has 4.8 dB insertion loss and 7% modulation depth, works as a mode-locker [28]. To achieve wide wavelength tuning range while maintain a strong filter effect, the $L_{PMF}$ of the filter section is set to 21 cm (L1), which corresponds to a 30 nm FSR. Splicing points before and after the filter section are 45$^\text {o}$ angle-spliced to ensure a maximum filter effect. Since the total length of PM fiber between the first angle splicing point and the polarizer determines the FSR of a Lyot filter, an extra piece of PMF with length L3 is 90$^\text {o}$ angle-spliced with the pigtail fiber with length L2 from the PM-TIWDM to compensate for the walk-off (L2=L3=85 cm). Therefore, the total length of the cavity is 5.5 m, and the laser operates in the conventional soliton regime. The PMF of the filter section is coiled around a PZT fiber stretcher (CoreMorrow H01.80). When the stretcher expands, it will apply a tensile strain to the filter section and tune the output wavelength. A waveform generator is used to drive the stretcher via a high voltage PZT driver amplifier.
Fig. 2. Experiment setup. LD: 980 nm pump laser diode. PM-TIWDM: Polarization-maintaining isolating wavelength–division multiplexing coupler with 50% output. CNT-ML: Carbon nanotube mode-locker. PM-EDF: Polarization maintaining Erbium doped fiber.
3.2 Wavelength-tunable operation
The relationship between the PZT driver’s output voltage and the microstrain induced by the PZT fiber stretcher is shown in Fig. 3. There is a linear relationship between the induced strain and the PZT voltage. At the maximum voltage of 150V, 1,204$\mu \epsilon$ can be induced.
Figure 4(a) shows the optical spectra of different voltage settings, measured with an optical spectrum analyzer (YOKOGAWA, AQ6375), the resolution bandwidth (RBW) is 0.05 nm. Figure 4(b) shows the relationship between the wavelength of the output light and the voltage of the PZT driver. The output voltage of the PZT driver is set to be in the 3V-67.5V range. Such voltage range corresponds to one wavelength-sweeping cycle. In the case of a static measurement, different DC voltage is applied to the PZT fiber stretcher, and the center wavelength of the output pulse shifts monotonically and linearly. Note that such tuning relationship is achieved when the environmental temperature is under control. Otherwise, the calibration is not accurate since temperature changes can also cause a wavelength shift [14].
Fig. 4. Static measurement of wavelength-tunable output. (a) Optical spectrum of wavelength tuning output. (b) Relationship between the center wavelength of the laser output and the PZT driver’s output voltage.
3.3 Fast wavelength sweeping
The fast wavelength sweep process is also investigated. Figure 5(a) shows the optical spectrum taken with max-hold mode and 0.05 nm RBW when the output wavelength is swept at 100 Hz. The wavelength tuning range is 27 nm. The spectrum is exceptionally flat, and no continuous wave peaks or noise are observed. Figure 5(b) shows the radio-frequency (RF) spectrum measured with an RF spectrum analyzer (RIGOL, DSA832) during the 100 Hz wavelength sweeping process with a RBW of 100 Hz. These figures prove that the laser mode-locking operation remains stable during the fast wavelength-sweeping process.
Fig. 5. Measurement of fast wavelength-swept output. (a) Optical spectrum measured in the max-hold mode at 100 Hz wavelength sweeping. (b) RF spectrum at 100 Hz wavelength sweeping.
The resonance frequency of the PZT fiber stretcher is 734 Hz. For safety and performance stability reasons, the frequency limit of the PZT fiber stretcher driver is set to 500 Hz. In order to investigate the characteristics of the proposed laser, the PZT fiber stretcher was driven with a sinusoidal wave at various frequencies and voltage ranges.
Figure 6 shows the optical spectrum measured in the max-hold mode when the PZT fiber stretcher was driven with sinusoidal waves at different wavelength-swept rates. The tuning ranges of 100 Hz, 300 Hz, and 500 Hz sweeping operations are 27 nm, 19 nm, and 13 nm, respectively. Figure 7 is the RF spectrum measured at a stationary state and different wavelength-swept rates. It shows that at different wavelength-swept rates, the stability of the laser is not affected. The tuning range shortens at higher wavelength-swept rate. There are several reasons that may cause this phenomenon. The first reason is that the effect of pulse shaping dynamics is involved. When the transmittance peak of the Lyot filter shifts dynamically, the pulses in the laser are deformed by the optical filter, while the CNT-ML tries to maintain the mode-locking operation. The second possible reason is due to the limited frequency response of the fiber stretcher. The third reason may be the stress relaxation and creep of the fiber. It is an interesting topic to further study the reasons behind this phenomenon. Therefore, in practical application scenarios, the wavelength tuning speed is not only limited by the maximum driving frequency of the PZT fiber stretcher, and trade-offs are required.
Fig. 6. Optical spectra measured in the max-hold mode when PZT fiber stretcher was driven with sinusoidal waves. Orange: 100 Hz. Purple: 300 Hz. Green: 500 Hz.
Fig. 7. RF spectrum measured with 100 Hz RBW at different wavelength-swept rate. (a) Static measurement (0 Hz), (b) 100 Hz, (c) 300Hz, (d) 500Hz.
4. Conclusion
We have proposed a novel wavelength tuning mechanism using a strain-controlled all-PM fiber Lyot-filter. For the first time, such a wavelength-tuning mechanism has been reported to the best of our knowledge. The wavelength tuning mechanism is a two-stage magnification control system. Around the 1550 nm wave band, the total magnification factor is 1,100. As a result, this tuning mechanism is 43 times more efficient than that achievable by other strain-controlled approaches, such as a tunable FBG filter. An all-PM all fiber fast wavelength-swept mode-locked laser is demonstrated. When the PZT fiber stretcher is driven by DC voltage at a range of 3V to 67.5V correspond to 24$\mu \epsilon$ to 542$\mu \epsilon$ induced strain, the laser has a 27 nm tuning range from 1540 nm to 1567 nm. Limited by the operating driving frequency of the PZT fiber stretcher, the maximum wavelength-swept rate is 500 Hz and therefore, the maximum wavelength tuning speed is 13,000 nm/s. The pulse shaping dynamics, the wavelength tuning range decreases to 19 nm at the 300 Hz swept rate and 13 nm at the 500 Hz swept rate. This phenomenon needs further study. Nevertheless, the laser can sweep a wide wavelength range within milliseconds and generate sub-picosecond pulses, which is ideal for coherent Raman microscopy. Moreover, the tuning mechanism has potential to be utilized in different wavelength bands, such as 1000 nm. By optimizing the length of the Lyot filter, it is possible to achieve a broader wavelength tuning range. Furthermore, the laser cavity is extremely simple and constructed in an all-PM fiber configuration. Therefore, it is highly robust to environmental perturbations. Within a 24-hour stability test, the mode-locking output had no significant change, and the wavelength sweeping process can repeatedly work for hours. Therefore, this laser is a promising light source for practical field deployment outside of laboratory.
Funding
Japan Society for the Promotion of Science (18H05238, 22H00209); Core Research for Evolutional Science and Technology (JPMJCR1872).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data that support the findings of this study are available from the corresponding author upon reasonable request.
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