We report here, to the best of our knowledge, for the first time high-efficiency laser wavelength conversion from 1.5 µm band to 1.7 µm band in deuterium-filled hollow-core photonic crystal fibers by rotational stimulated Raman scattering (SRS). Due to the special transmission properties of this low-loss hollow-core fiber, the ordinary dominant vibrational SRS is suppressed, permitting efficient conversion to the rotational stokes wave in a single-pass configuration pumped by a fiber amplified and modulated tunable 1.55 µm diode laser. Using proper pump pulse energy and gas pressure, the power conversion efficiencies over the whole output laser wavelength range from 1640 nm to 1674 nm are higher than 48%. And the maximum Raman conversion efficiency of 61.2% is achieved with 20 m fiber and 20 bar deuterium pressure pumped at 1540 nm, giving a maximum average power of about 0.8 W (pulse energy of 1.6 µJ). This work points to a new way for engineerable and compact fiber lasers operation at 1.7 µm band, which has significant applications in biological imaging, laser medical treatment, material processing and detecting.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Gas SRS has been proven to be an effective way to extend and enrich existing laser wavelengths since it was first reported in 1963 . Historically, gas SRS has been plagued by high pump power due to short interaction distance and low interaction intensity in gas cells, as well as incidental conversion to other unwanted wavelengths, resulting in low efficiency conversion to the desired wavelength. The emergence of hollow-core fiber (HCF) opens new prospects for the interaction of light and gases, especially by SRS . Since the SRS of hydrogen in air-core fibers was first reported in 2002 , with the fast development of HCFs [4–9], a large number of experiments on gas-induced Raman scattering based on HCFs have been reported [10–20]. It provides a perfect platform for the interaction between gas and laser due to its easy control of the transmission spectrum and the long interaction distance. A HCF with low loss transmission for the pump light and wanted Stokes light can be easily obtained by designing the HCF's micro-structure, while other Raman lines are located in the stop band, thereby greatly improving the conversion efficiency to the desired Stokes wave [10,12,15,19]. In HCFs the overlap between the core mode with the cladding material is negligible and the field in the silica material is at least an order of magnitude smaller than the peak filed in the fiber hollow core, giving a higher damage threshold and making it ideal to operate at higher output power .
Fiber lasers at 1.7 µm band have important applications in medical, material detection, material processing, bioimaging, and generation of mid-infrared laser output , and have received great attention in the past years [22–25]. Normally, there are three ways to generate this band of fiber lasers: by thulium-doped fiber , special ion-doped fiber , and nonlinear effects based on Raman frequency shift . However, 1.7 µm is at the edge of the gain band of thulium-doped fibers, and the gain is low, making it very difficult to generate efficient laser emission at this band. And the special ion-doped fiber is in the initial stage of development, and the manufacturing technique is still immature. The gas SRS in HCFs can provide a simple and efficient way for generation of near-infrared light in 1.7 µm band if we use proper HCFs and active Raman gases.
Here we report the first demonstration of an efficient laser wavelength conversion from 1.5 µm band to 1.7 µm band in a deuterium-filled hollow-core photonic crystal fiber (HC-PCF) by rotational SRS. Because of the special transmission loss spectrum of the used HC-PCF and the proper Raman frequency shift, highly efficiency laser emission over the range of 1640 nm to 1674 nm (Raman power conversion efficiency >48%) is generated by using a homemade fiber amplified and modulated tunable 1.55 µm diode laser as the pump source. The stokes conversion under different pump wavelengths, gas pressures and repetition frequencies is experimentally studied in details. A maximum Stokes power of about 0.8 W (pulse energy of 1.6 µJ) with a maximum Raman conversion efficiency of 61.2% is obtained using 20 m fiber and 20 bar deuterium pumped at 1540 nm when the repetition frequency is 500 kHz. The Raman conversion process is also investigated by analyzing the pump and Stokes pulse shapes at different repetition frequencies and pump powers. This work opens a new opportunity for efficient tunable laser sources operating at 1.7 µm band.
2. Experimental setup
Figure 1 shows the experimental setup for D2 rotational SRS in HC-PCFs. A fiber coupler with a measured coupling ratio of 99.16:0.84 is spliced to the output end of the pump laser to monitor the pump power in real time. The signal port (SMF-28e) is spliced to the HC-PCF directly, which can make the system compactly and stability. Comparing to space coupling method, this method can avoid the damage to the fiber end face by high power laser. As SMF-28e has nearly the same mode field diameter and numerical aperture with the HC-PCF, the coupling efficiency of the fusion splicing can be very high in theory. However, due to the inevitable damage to the microstructure during the fusion process, the real coupling efficiency is estimated to be about 72% (which can be further enhanced by improving the fusion technique) by subtracting the fiber loss from the ratio of output power to input power (the HC-PCF is 20 m). The other end of the HC-PCF is sealed into a gas chamber, through which we can vacuum the system or fill it with D2. A convex-plane lens is placed after the output window to collimate the output light. A long pass filter (LPF1600, transmission ∼ 95% > 1600 nm) and a band pass filter (BPF1550-40, transmission ∼61% at 1535 nm, ∼67% at 1565 nm) are used here to detect the Raman power and the residual pump power with a power meter. The inset figure in Fig. 1 shows the cross section and attenuation spectrum of the HC-PCF. The HCF used in the experiment is produced by NKT Photonics (HCF-1550-02), and the losses of the pump lines and Raman lines are about 0.016 dB/m and 0.02 dB/m.
The pump laser is a homemade fiber amplified and modulated tunable 1.5 µm diode laser (CobriteDX1, wavelength range from 1527 nm to 1567 nm), as shown in Fig. 2(a). The fiber amplifier consists of three stages of erbium-doped fiber amplifiers (EDFAs), an acousto-optic modulator (AOM, Gooch & Housego, Fiber-Q) and a tunable fiber filter. The AOM is used to modulate the signal with a real pulse width of ∼12 ns and the repetition frequency of the pulse can be modulated. The tunable fiber filter is used to inhibit the amplified spontaneous emission (ASE). Figure 2(b) shows the maximum output spectra of the pump source operating at different wavelengths, and it can be seen that the ASE is inhibited well. Figure 2(c) plots the evolution of the maximum output power of the tunable pump source with the wavelengths. Due to the characteristics of the EDFA, the output power of the pump laser decreases approximately linearly with increasing wavelength. The abnormal power at 1545 nm is due to the lower insertion loss of the tunable filter than other wavelengths.
3. Experimental results and discussion
Figure 3 shows the output spectra at different pump wavelengths, with gas pressure of 20 bar and pump repetition frequency of 500 kHz. It can be seen that the dominant vibrational SRS (Raman frequency shift 2987 cm−1) is suppressed because of the narrow transmission band of the HC-PCF, which leads to the efficient conversion to rotational SRS (Raman frequency shift of 414.5 cm−1). For each pump wavelength, there is only one rotational stokes line (ortho deuterium Raman shift of 414.5 cm−1 from rotational energy level J = 2 to J = 4) in the transmission band. That is because other rotational Stokes lines are at the edge of the HCF's transmission band, as well under 500 kHz repetition frequency, the peak power of the pump pulse doesn't achieve the threshold of other rotational stokes. There is no anti-stokes lines in the spectra, just because the anti-stokes lights are outside of the transmission band of the HC-PCF. Thus, we can get pure first-order rotational stokes wave of D2. As the wavelength of the pump light is tuned from 1535 nm to 1565 nm, the wavelength of the Raman light will change from 1640 nm to 1674 nm. Figures 3(b) and 3(c) show the near-field patterns of 1540 nm pump wave (which is measured after HC-PCF and without Raman conversion) and its Raman light of 1645 nm, measured by using a 20× microscope objective and a HgCdTe infrared camera (Xenics MCT-2327). It can be seen that due to the beam cleanup effect of the SRS in fibers, the near-field pattern of the Stokes wave, which is close to single-mode, is better than that of the pump laser .
Figure 4 shows the variation of Raman power (through the LPF 1600 measured by a power meter) and efficiency with coupled pump power at different pump wavelengths and different D2 gas pressures. It can be seen that the threshold of the average power is about 500 mW (∼100 W peak power), when the pump power is higher than the threshold, Raman power will increase nearly linearly with the coupled pump power. The slope efficiency at different pump wavelengths are nearly the same. It may because the losses of the HC-PCF at these wavelengths are nearly the same and the Raman gain does not change much with wavelengths. The maximum Raman power decreases as pump wavelength increases, which is due to the characteristics of EDFAs that the maximum pump power also decreases with the increase of wavelength as shown in Fig. 2(c). A maximum output power 0.8 W (pulse energy ∼0.16 µJ) and the maximum light conversion efficiency 61.2% is gotten when pumped at 1540 nm. Figures 4(c) and 4(d) show the variation of Raman power and efficiency with coupled pump power at different D2 gas pressure when the pump wavelength is 1540 nm. It can be seen that the threshold at 5 bar gas pressure is about 530 mW and as pressure increases, the threshold gradually decreases due to higher Raman gain at higher pressure. Raman power will increase nearly linearly with coupled pump power after the threshold. From the experiment, we get the maximum output power at 20 bar gas pressure. At lower pressure, the Raman threshold is higher, resulting in less pump light is absorbed. While at higher pressure, the decreasing of the conversion efficiency to the first order Stokes wave is most possibly due to the occurring of the cascaded Raman scattering, as shown in Figs. 6(c) and (e).
We measured the pulse shapes and optical spectrum at different repetition frequencies of 500, 200 and 100 kHz (corresponding to the maximum coupled peak power of 220, 550, 1100 W respectively). Figure 5 shows the pulse shapes of Raman light (1645 nm, through LPF1600) and residual pump light (1540 nm, through BPF1550-40) at different pump power of the fiber amplifier with three different repetition frequencies. A fast photodetector (EOT ET5000, bandwidth 12.5 GHz) and a broadband oscilloscope (Tektronix MDO3104, bandwidth 1 GHz, sample rate 5 Gs/s) is used to detect the pulse shapes. From Fig. 5, we can see that when the pump power is higher than the threshold, the central part of the pump pulse will be converted to Raman light, which leads to the sunk of the residual pump light. As shown in Figs. 5(a) and 5(b), as the pump power increased after the threshold, the Raman pulses increase with the pump power but the pulse shapes of the residual pump is nearly unchanged, only the interval between two sides increased. This is because the threshold is unchanged, and only the pulses whose peak power is beyond the threshold undergo the SRS process, which will convert to the Raman pulse. As the repetition frequency decreases, the peak power of the pump also increases, which achieves the threshold of cascade Raman lines, leading to the conversion from 1645 nm to its rotational stokes lines. The conversion process is reflected in Fig. 5(c) and (e) as the sunk at the middle of the pulse. Figures 5(c) and 5(d) show the pulse shapes at 200 kHz repetition frequency, at this frequency the conversion process starts appearing. Figures 5(e) and 5(f) show the pulse shapes at 100 kHz repetition frequency, at this frequency the conversion point is advanced compared with Fig. 5(c) because of the higher peak power, and the middle part of the residual pump light has increased shown in Fig. 5(f). We consider it that there has not only one cascade Raman lines at this frequency, which may cause the inhabitation of the generation of 1645 nm Raman light.
Figures 6(a), 6(c), and 6(e) show the output spectra at the repetition frequencies of 500, 200 and 100 kHz when pumped at 1540 nm at maximum power. It can be seen that when the repetition frequency is 500 kHz, the peak power is just higher than the threshold of the first-order stokes, and only 1645 nm Raman light can be seen. As the peak power increases and two new stokes lines are observed when the repetition frequency is 200 kHz, which are 1765nm (the first-order ortho rotational stokes of 1645 nm, Raman shift of 414.5 cm−1 from rotational energy level J = 2 to J = 4) and 1695 nm (the first order ortho rotational stokes of 1645 nm, Raman shift of 179 cm−1 from rotational energy level J = 0 to J = 2). As the repetition frequency continue decreases to 100 kHz, 1730nm Raman light (the first order para rotational stokes of 1645 nm, Raman shift of 297.4 cm−1 from rotational energy level J = 1 to J = 3) is obtained. This is because as the peak power increased, it has been achieved the threshold of other rotational stokes lines, which is influenced by their Raman gain. The inset pictures in Figs. 6(a), 6(c), and 6(e) show the pulse shapes of the Raman light, residual pump light and pump light at these repetition frequencies, which indicate the conversion process stated above. Figures 6(b), 6(d), and 6(f) show the variation of Raman power, residual pump power and efficiency with coupled pump power at 500, 200 and 100 kHz filled with 20 bar D2 when 1540 nm pumped. At 500 kHz in Fig. 6(b), the Raman power increased after the threshold, and after this point the residual pump power will decrease as the first-order rotational stokes (1645 nm) is generated. At 200 kHz in Fig. 6(d), the peak power increases and the first-order rotational stokes start to converse to its cascade rotational stokes (1695 nm and 1765nm). There has a point after that the Raman power decreases because of the generation of cascade Raman lines of the first-order rotational stokes, as for the residual pump power, after the threshold it decreases first and then increases, which may duo to the generation of other Raman lines inhibiting the conversion from 1540 nm to 1645 nm. At 100 kHz in Fig. 6(f), there has more rotational Raman lines (1730nm) than other two repetition frequencies, the inhabitation of the conversion from 1540 nm to 1645 nm will be stronger, leading to the increasing of residual pump power. A maximum average power of about 0.8 W (pulse energy of 1.6 µJ) is obtained and the Raman conversion efficiency can achieve 61.2% with repetition frequency of 500 kHz.
The linewidth of the 1540 nm pump and 1645 nm Stokes light are measured using a scanning Fabry Perot Interferometer (Thorlabs SA210-12B, free spectral range of 10 GHz), which are estimated to be about 0.27 GHz and 0.58 GHz respectively at the maximum pump power. Comparing with solid-core rare earth doped fiber lasers, this kind of fiber gas Raman lasers have more potential advantages in high power meanwhile narrow linewidth emission, which is required in many applications.
We have firstly demonstrated here, high efficiency laser wavelength conversion from 1.5 µm band to 1.7 µm band in deuterium-filled hollow-core photonic crystal fiber by SRS. Pumped by a homemade fiber amplified and modulated tunable 1.55 µm diode laser, efficient Raman light from 1640 nm to 1674 nm is obtained by rotational SRS of deuterium. The maximum Raman power of 0.8 W (pulse energy of 1.6 µJ), with a maximum Raman conversion efficiency of 61.2% is obtained with 20 m fiber and 20 bar gas pressure pumped at 1540 nm. Preliminary theoretical analysis shows that by reducing the fusion loss, shortening the length of the HC-PCF and optimizing the pump fiber amplifier’s performance (pulse width, repetition frequency, peak power, et al.), the Raman laser conversion efficiency and output power can be further improved. This research proves an efficient way for engineerable and compact fiber lasers at 1.7 µm band.
Natural Science Foundation of Hunan Province (2019JJ20023); National Natural Science Foundation of China (11974427).
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