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Abstract
We developed a diode-pumped, mode-locked polycrystalline Cr:ZnS oscillator using single-walled carbon nanotubes as a saturable absorber. The oscillator exhibits self-start mode-locking operation, generating sub-100 fs pulses with an average power of 300 mW. We found a unique feature in which the intensity noise originating from relaxation oscillation is suppressed by inherent second harmonic generation in polycrystalline Cr:ZnS. The observed noise suppression is reproduced by a theoretical model that includes an instantaneous nonlinear loss.
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1. Introduction
Cr2+-doped ZnS/ZnSe crystals exhibit broad emission bandwidths in the 2-3 µm wavelength range (centered at 2.4 µm) and good thermo-optical and thermomechanical properties and are promising as ultrashort-pulsed laser gain media in the mid-infrared region [1]. To date, sub-100 fs pulse generation has been successfully achieved for Cr:ZnS/ZnSe lasers based on the use of various mode-locking mechanisms [2–5]. Femtosecond Cr:ZnS/ZnSe-based lasers show great potential for direct mid-infrared frequency comb generation [6], high harmonic generation [7], and vibrational spectroscopy [8]. They are also ideal pump sources for nonlinear optical frequency down conversion to cover the “molecular fingerprint” region [9–10].
For high-precision applications of frequency comb generation and vibrational spectroscopy/microscopy, suppression of intensity noise is essential. Wang et al. [11] analyzed the relative intensity noise (RIN) of a Kerr-lens mode-locked (KLM) Cr:ZnSe oscillator and found that the intensity noise originating from relaxation oscillation (RO) of the pumping Er-fiber laser, located at approximately 100 kHz, is transferred to the KLM Cr:ZnSe laser. The integrated RIN in a frequency range from 10 Hz to 10 MHz was determined to be 0.3%. Nagl et al. [12–13] reported a KLM Cr:ZnSe oscillator directly pumped by a watt-class InP diode single emitter. This pump source offers great advantages in terms of affordability, high wall-plug efficiency, compactness, and low RIN. Although intensity noise originating from the RO of the Er-fiber laser was absent, the RIN spectrum showed a plateau at several hundred kHz, which was attributed to the RO of the Cr:ZnSe oscillator [12,14].
Here, we report a directly diode-pumped, mode-locked polycrystalline Cr:ZnS oscillator that utilizes a single-walled carbon nanotube (CNT) saturable absorber. The oscillator exhibits self-start mode-locking operation, generating 83 fs pulses with an average output power of 300 mW. Most importantly, we find a unique feature in which the intensity noise originating from the RO of the Cr:ZnS oscillator is suppressed by inherent second harmonic generation (SHG) in polycrystalline Cr:ZnS. The observed suppression of RO is reproduced by a theoretical model that takes into account an instantaneous nonlinear loss of similar magnitude as the experimental loss. The resultant integrated RIN is 0.078% and 0.16% for a frequency range of 1 kHz ‒ 10 MHz and 10 Hz ‒ 10 MHz, respectively. Suppression of RO noise by nonlinear loss has been previously demonstrated in continuous wave (CW) solid-state lasers by insertion of a nonlinear crystal into the laser cavity [15–16]. We successfully demonstrate suppression of RO noise in a mode-locked solid-state laser, taking advantage of the nonlinearity of the gain medium itself rather than introducing an extra nonlinear crystal into the laser cavity.
The remainder of this paper is organized as follows: In Section 2, we show the experimental setup. In Section 3, we present the pulse characteristics, RIN properties, and our theoretical analyses for the suppression of RO noise. Finally, we present our conclusion in Section 4.
2. Experimental setup
The experimental setup for the directly diode-pumped, CNT mode-locked Cr:ZnS oscillator is depicted in Fig. 1. The output from each of the two InP laser diode single emitters of 3W level (centered at 1650 nm, SemiNex Corp.) is collimated in the fast axis (perpendicular to the plane of paper) by a convex lens with f = 4.51 mm, collimated in the slow axis (within the plane of paper) by additional cylindrical lenses (f = -30 mm and 300 mm for C1 and C2, respectively), and combined by a polarization beam splitter (PBS). The combined pump beam is deflected by two gold mirrors and focused by a planoconvex lens with f = 60 mm, which theoretically leads to a beam diameter of 62 µm in the fast axis and 100 µm in the slow axis inside the laser cavity. The available maximum pump power at the focus is 5 W.
Fig. 1. Optical setup for the diode-pumped, CNT mode-locked Cr:ZnS laser: LDs, InP laser diode single emitters, C1’s and C2’s (cylindrical lens 1’s with f = -30 mm and cylindrical lens 2’s with f = 300 mm, respectively), CMs, chirped mirrors for third-order dispersion compensation, CNT/CaF2, a single-walled CNT film deposited onto a CaF2 plate, OC, and output coupler.
The anti-reflection-coated 5 mm long polycrystalline Cr:ZnS crystal with a doping concentration of 5 × 1018 cm-3 (IPG Photonics) is mounted onto a copper heatsink that is at a constant water cooling temperature of 20 °C. The laser cavity consists of four concave mirrors with a radius of curvature of 100 mm, two plane chirped mirrors (CMs) used for third-order dispersion compensation, and a wedged fused silica output coupler (OC) with a 10% transmittance. There are also two types of bulk plates (a 3 mm sapphire plate and a 2 mm CaF2 plate) used for group delay dispersion (GDD) compensation in the cavity. The net GDD in the cavity is approximately -550 fs2 (GVD at 2.35 µm: 123 fs2 mm-1 for Cr:ZnS, -230 fs2 mm-1 for sapphire, and -47 fs2 mm-1 for CaF2). A single-walled CNT film with resonant absorption centered at a wavelength of 2.4 µm is deposited onto the surface of the CaF2 plate. Details for the single-walled CNT film and the two CMs are described in Ref. [17]. With the CNT device inserted into the cavity, we can easily initiate stable mode locking by adjusting the position of the CNT device and by adjusting the angle of the concave end mirror. The spectral intensity of the laser output is measured by using a Czerny-Turner monochromator and an InGaAs photodiode (PD, Thorlabs, DET05D2). The interferometric autocorrelation (IAC) of the output pulse is measured via two-photon absorption (TPA) using an InGaAs PD (Thorlabs, DET10C2).
The intensity noise properties of the diode-pumped mode-locked oscillator are measured using the following procedures. First, the mode-locked output is attenuated by a neutral density filter to be less than 7 mW. Second, a possible second harmonic component is blocked by a germanium (Ge) plate, which is placed at a Brewster angle for the fundamental frequency. Here, the second harmonic component can be generated via random quasi phase matching (RQPM) in polycrystalline Cr:ZnS [18–20]. Then, the beam is focused onto an InGaAs PD (Thorlabs, DET05D2) by a lens with f = 100 mm. The signal from the PD is sent to a radio frequency spectrum analyzer (Agilent, E4402B), where the intensity fluctuation is analyzed after passing through a low pass filter with a 23 MHz cutoff frequency. Limited by the frequency range of the spectrum analyzer, a lower frequency RIN measurement (<10 kHz) is performed by using an oscilloscope (Rohde & Schwarz, RTO-1014). The RIN is measured with a resolution bandwidth (RBW) of 100 Hz and 300 Hz in the range of 10 kHz ‒ 100 kHz and 100 kHz ‒ 1 MHz, respectively. Finally, these RIN data are stitched together.
3. Results
Figure 2(a) displays the slope efficiencies for the CW oscillation and mode-locked oscillation with a repetition rate of 140 MHz. When the oscillator is optimized for CW output, the 2 mm CaF2 plate with CNT film is replaced by another 2 mm CaF2 plate without the CNT film. The slope efficiency of the CW oscillation is 24%, with a maximum output power of 724 mW at a pump power of 4.1 W. The mode-locked output power represents a slope efficiency of 20%. When the pump power is lower than 3.63 W, Q-switched mode-locking occurs. The maximum output power under CW mode locking is 324 mW at a pump power of 4.1 W. For the pump power between 3.63 W and 4.1 W, we observe self-start CW mode-locking without multipulse phenomenon. A further increase in the pump power leads to degraded spatial overlap between the two combined pump beams and to unstable mode locking. Although it is possible to readjust the spatial overlap between the two pump beams, we present only the results taken without such secondary adjustment.
Fig. 2. (a) Pump power dependence of the output power for the CW oscillation (square, slope efficiency of 24%) and that for the mode-locked oscillation (circle, slope efficiency of 20%). (b) The output power of the mode-locked oscillator measured over an hour (the inset shows the beam profile). (c) The spectral intensity of the mode-locked pulse. The FWHM bandwidth wavelength and frequency is 84.2 nm and 4.6 THz, respectively. (d) The TPA IAC trace, from which the pulse duration is estimated to be 83 fs assuming a sech2-shaped intensity profile.
The long-term stability of the mode-locked output power in open air, measured by a thermopile power meter (Ophir 3A), with a detector response time of 1.8 s and a logging rate of 15 points/sec), is shown in Fig. 2(b). The mean value and standard deviation are 285.4 mW and 1.63 mW, respectively, representing good power stability with a root mean square (RMS) value of 0.57% over three hours. The output beam has a nearly Gaussian spatial profile, as shown in the inset of Fig. 2(b). The mode-locked pulse spectrum at an output power of 280 mW is shown in Fig. 2(c). The wavelength and frequency of the spectral full width at half maximum (FWHM) is 84.2 nm and 4.6 THz, respectively. Figure 2(d) displays the measured IAC trace, from which the pulse duration is evaluated to be 83 fs in FWHM by assuming a sech2-shaped intensity profile. Here, we note that the pulse in the laser cavity acquires a net GDD of -610 fs2 before reaching the InGaAs PD of the IAC setup, in the course of transmission through a 6.35 mm thick fused silica OC, a 0.3 mm thick Ge window placed in front of the IAC setup to block possible shorter wavelength components, and a 2 mm thick CaF2 beam splitter placed inside the IAC setup. The time-bandwidth product of 0.38 is larger than but not far from the value of 0.315 obtained for the Fourier transform-limited sech2-shaped pulse.
Figure 3 shows the measured RIN power spectral densities (PSDs) and the spectrally integrated RIN values. The RIN of the background (BG) would stem from the PD and the spectrum analyzer, considering the calculated shot noise level of -151.7 dBc/Hz. The spectrally integrated BG RIN from 10 Hz to 10 MHz is 0.06%. Both the RIN PSDs for the LDs and the mode-locked laser show a f-1 structure [11,21] with obvious peaks observed below several hundred Hz, which are mainly due to acoustic noise, mechanical vibrations and 50-Hz AC power lines [22]. The RIN falls and becomes smooth from 1 kHz to 10 kHz. From 10 kHz to 10 MHz, the RIN for LDs tends to be smooth with occasional lines. One obvious line located at approximately 100 kHz arises from the LD drivers.
Fig. 3. RIN PSD (left) and integrated RIN (right) over a frequency range from 10 Hz to 10 MHz. LDs: RIN for the combination of two LDs. ML: RIN for the mode-locked Cr:ZnS laser. BG: RIN for the photodiode and spectrum analyzer (the photodiode and oscilloscope at < 10 kHz).
At frequencies above 1 kHz, the RIN of the mode-locked Cr:ZnS laser is substantially suppressed compared with that of the LDs to the same level as the BG RIN. The suppression is attributed to the time averaging effect of the laser gain in the Cr:ZnS crystal whose upper state lifetime is approximately 5 µs at room temperature [1]. The integrated RIN is 0.078% for a frequency range of 1 kHz ‒ 10 MHz, 0.11% for 100 Hz ‒ 10 MHz, and 0.16% for 10 Hz ‒ 10 MHz. Here, we note that the integrated RIN increases at a frequency of around 100 Hz due to mechanical vibrations in the lab, which can be avoided with careful mechanical isolation. There is no bulge observed at approximately several tens of kHz, which is often observed for Er-fiber lasers. There is a plateau-like structure at frequencies of 100 kHz ‒ 1 MHz, which originates from the strongly damped RO of the Cr:ZnS oscillator. Similar plateau-like structures have been observed in mode-locked solid-state lasers [12,14].
In our initial experiments, a prominent RO peak at approximately 720 kHz was always observed. The RO is an intrinsic characteristic of class-B lasers (where the population inversion lifetime is longer than the cavity photon lifetime) and is often located in the frequency range from kHz to MHz [23–25]. The RO may be excited by some unavoidable factors, such as vacuum noise and fluctuation of the intracavity loss [24,26,27]. In our experiments, although the RO peak appearing at 720 kHz is reduced to some extent by careful adjustment of the diode pump power, it persists, as shown in Fig. 4(a). Here, we utilize the inherent SHG generated by RQPM in polycrystalline Cr:ZnS to suppress the RO peak. The polycrystalline Cr:ZnS is gradually shifted toward the focus of the oscillating mode (as shown by an arrow in Fig. 1) to enhance the SHG efficiency. With a shift of up to 0.4 mm, the RO peak is eliminated, as shown in Figs. 4(b) – (d). Note that we maintain the laser output power at approximately 295 mW (or the intracavity power at 2.95 W) by adjusting the diode pump power while shifting the Cr:ZnS gain media. We monitor the second harmonic (SH) power by measuring the optical power reflected from the sapphire plate (see Fig. 1) and transmitted through a shortpass filter (Spectrogon, SP1550). The measured SH spectrum is shown in the inset of Fig. 4(d).
Fig. 4. Suppression of the RO peak by enhancing the SHG efficiency in polycrystalline Cr:ZnS. The measured RIN PSD for the mode-locked Cr:ZnS laser in the frequency range marked by the black arrow in Fig. 3 is shown as red lines for varied SH power (the estimated intracavity SH power): (a) 1.22 mW (20.12 mW), (b) 1.49 mW (24.56 mW), (c) 1.6 mW (26.4 mW), and (d) 1.96 mW (32.32 mW). The black lines display theoretical fittings. The inset of (d) represents the measured SH spectrum.
At the initial position far from the focus of the oscillation mode, the measured SH power reflected from the sapphire plate is 1.22 mW. Considering the reflectance at the sapphire plate of 12.5% and the transmission loss of the concave mirror of 3%, the SH power generated within a single pass through the Cr:ZnS crystal is estimated to be 10.06 mW and that generated within one round trip is 20.12 mW. Here, we regard the SH component generated through the RQPM process as nonpolarized even though the 2.4 µm fundamental wave is linearly polarized [18,28]. For the case of Fig. 4(a), the intracavity SHG efficiency is calculated to be 0.68% using an intracavity SH power of 20.12 mW. When the Cr:ZnS crystal is shifted by 0.15 mm toward the focus, the intracavity SHG efficiency (power) is enhanced to 0.82% (24.56 mW), and the RO peak height is reduced, as shown in Fig. 4(b). As the crystal is moved further toward the focus, the RO peak is substantially suppressed at an SHG efficiency of 0.89% and eliminated at an SHG efficiency of 1.11% (an intracavity SH power of 32.32 mW), as shown in Figs. 4(c, d). The total suppression of the RO peak due to the 0.4 mm shift of the gain media is 10 dB. Note that a stable pulse train typical for CW mode locking is observed throughout the measurements shown in Figs. 4(a)–(d).
4. Discussion
The mechanism of RO suppression can be simply explained as SHG introducing an instantaneous nonlinear loss to inactivate the interplay between population inversion and intracavity photon flux [15]. Based on this physical picture, we use transfer function theory [29–31] to reproduce the observed suppression of the RO peak. By considering the pump rate noise and the cavity loss fluctuation as the main contributors, the RIN can be expressed as follows [29]:
As shown in Fig. 4, the experimental RIN data are well fitted by the theoretical curves based on Eq. (1). Table 1 summarizes the SHG efficiencies for the experiments and numerical fittings for different crystal positions. Both the measured and fitted values represent the same trend: the RO peak is more strongly suppressed as SHG is enhanced. Small deviations in the SHG efficiency between experiment and theory can arise for the following reasons: First, third harmonic generation in the Cr:ZnS crystal can contribute to the suppression of RO in experiments, although it is not included in the theoretical model [3]. Second, the change in the oscillating beam diameter at the Cr:ZnS crystal is not taken into account in the theoretical model. The decrease in the oscillating beam diameter can introduce nonlinear loss through the self-focusing effect, although our intracavity peak power of approximately 250 kW is smaller than the self-focusing threshold of 400 kW in Cr:ZnS [33]. It should be noted that no change in the RIN PSD is observed at frequencies other than those relevant to RO when we shift the crystal position to enhance SHG. This fact, together with a good agreement with the theory, confirms that the SHG process works solely to suppress the excess RO noise by modifying the intracavity photon fluctuations.
Table 1. Measured and fitted SHG efficiencies for different positions of the laser crystal
5. Conclusion
We have developed a diode-pumped, mode-locked polycrystalline Cr:ZnS oscillator using a single-walled CNT saturable absorber. The oscillator exhibits self-start mode-locking operation, generating 83 fs pulses with an average power of 300 mW. We find that the RIN originating from the RO of the Cr:ZnS oscillator is suppressed by the inherent SHG in the Cr:ZnS crystal. This is the first demonstration of the RO suppression in solid-state lasers using inherent SHG. The observed RO suppression can be reproduced by a theoretical model with reasonable agreement for the SHG efficiencies. The resultant integrated RIN is 0.078% and 0.16% for a frequency range of 1 kHz ‒ 10 MHz and 10 Hz ‒ 10 MHz, respectively. The inherent noise suppression mechanism for the polycrystalline Cr:ZnS makes it stand out among other femtosecond laser gain media.
Funding
Japan Science and Technology Agency (Core Research for Evolutional Science and Technology JP20348765); Japan Society for the Promotion of Science (20H02651, 20J22067, 20K20556); Ministry of Education, Culture, Sports, Science and Technology (Q-LEAP JPMXS0118068681).
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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