A monolithic polarization maintaining fiber chirped pulse amplification system with 25 cm Yb3+-doped high efficiency media fiber that generates 62 µJ sub-400 fs pulses with 25 W at 1.03 µm has recently been demonstrated.
© 2015 Optical Society of America
Industrial applications of ultrashort pulse lasers are successfully demonstrated in many areas such as consumer electronics, automotive industry, medical industry, etc [1,2]. Monolithic fiber-optic chirped pulse amplification (CPA) systems [3,4] are highly attractive owing to the compactness, stability, reliability and thermal management enabled by fiber format .
Despite of all the advantages of monolithic fiber-optic CPA systems, ultrafast fiber laser systems are constrained in output pulse energy by the temporal pulse distortion caused by self-phase modulation (SPM). The magnitude of SPM is quantified by the B-integral, which is proportional to both the peak irradiance along the fiber amplifier and the propagation length and B-integral is defined as6]. These established approaches all operate in the near fundamental fiber mode (LP01) which suffers from bend-induced reductions in the mode size and increases in modal instability. Such laser assemblies are further complicated as they necessitate free-space coupling due to the challenges of direct fiber splicing, as well as large footprint. It has been demonstrated that these traditional LMA fiber limitations may be overcome by using a higher-order mode (HOM) fiber amplifier . However, the HOM gain fiber is still required to be quite long, and fabricating these amplifiers with mode converting long period gratings (LPGs) remains complicated.
In contrast, we have proposed using a high efficiency media (HEM) fiber amplifier to achieve the B-integral reduction by greatly shortening the required gain fiber length while maintaining reasonable gain and efficiency . Moreover, we have demonstrated a monolithic Er3+-doped HEM fiber CPA system for millijoule femtosecond pulse generation at 1.55 µm without suffering parasitic energy loss associated with ion clustering in more traditional glass fibers, e.g. silica . In this paper, we demonstrate the first monolithic Yb3+-doped HEM based fiber-optic polarization maintaining (PM) CPA system for high energy femtosecond pulse generation at 1.03 µm. A key aspect of our amplifier design is the complete absence of photodarkening degradation of the gain fiber. The HEM Yb3+-doped PM gain fiber was just 25 cm in length with an Aeff = 855 µm2. The resultant 1.03 µm wavelength monolithic PM fiber CPA laser architecture produced 62 µJ pulse energy with 380 fs pulse duration at ~400 kHz repetition rate which is close to 25 W average power.
2. Experimental setup
The LMA Yb3+ -doped HEM fiber was fabricated by AdValue Photonics Inc. using a rod-in-tube technique  with estimated 260 dB/m cladding absorption at 976 nm. Assuming a step-index profile, the Aeff for this fiber design is 855 µm2 (simulated by MODE Solutions). Two stress rods were introduced for polarization maintaining.
To build an amplifier using the Yb3+-doped LMA HEM fiber, the input splice between the silica mode field adapter fiber and the HEM fiber was optimized by minimizing both the splice loss and the observed back-reflection. Following splicing to the other end of the LMA HEM amplifier fiber, a 500 µm diameter end-cap was angle polished at 8° in order to minimize back reflection and avoid surface damage at the glass-to-air interface.
Light from two 976 nm multimode laser diode pump modules and the pre-amplified signal were combined in a single mode high power PM (2 + 1) × 1 tapered fiber bundle (TFB). The TFB output was fusion spliced to the Yb3+-doped HEM fiber amplifier. Finally, the assembled LMA Yb3+-doped HEM fiber amplifier was mounted on a water-cooled aluminum plate held at 22°C.
Figure 1 shows a high level schematic of our fiber femtosecond laser architecture at 1.03 µm which relies upon the standard CPA concept implemented in a monolithic, fiber-optic format. This architecture includes discrete, active feedback and control loops based on laser average power and pulse quality.
The mode-locked laser (MLL) developed by Raydiance is centered at 1.03 µm and its repetition rate is 23.6MHz. Environmental stability is verified by testing both self-start and output power of the MLL at different environmental temperature. Optical spectral bandwidth from the MLL is adjustable within the range of 7~11nm and average power is 1~2mW. Pulses generated from the MLL are temporally stretched to ~1 ns full width at half maximum (FWHM) by a chirped fiber Bragg grating based pulse stretcher. Output from the pulse stretcher is amplified by the first polarization maintaining (PM), single mode (SM) Yb3+ doped fiber amplifier (Preamp1) and the operating condition of the Preamp1 is optimized by doing a cutback experiment.
The pulse rate is reduced from 23.6 MHz to ~400 kHz using >35 dB extinction PM fiber acousto-optic modulator (AOM), labeled Pulse Picker in the figure. The second Preamp2 is installed right after PM fiber AOM. The straight 25 cm long LMA Yb3+-doped HEM fiber amplifier, labeled Booster, is spliced to Preamp2. Finally, a free-space Treacy, single grating compressor is used to compress the pulses from the LMA Yb3+-doped HEM amplifier to 380 fs, as characterized by intensity autocorrelation.
It is important to mention the design principle of both pulse stretcher and compressor in industrial grade monolithic fiber optic CPA system. Despite of all the advantages of monolithic fiber optic laser system, its constraint has been the scaling of output energy, which is not limited by stored energy inside fiber amplifier but by detrimental nonlinearities which is a source of pulse distortion, throughput reduction, etc. Preferable approach to increase output energy is to increase the amount of stretched pulse duration, in order to reduce the amount of nonlinearities. Two general approaches to generate longer stretched pulse are to increase spectral bandwidth and dispersion of pulse stretcher/pulse compressor. However the concept of industrial laser design limits the footprint of pulse compressor for efficient packaging, which forces the dispersion of pulse compressor to include higher order dispersion terms. Other design parameters such as beam quality, alignment tolerance, environmental stability, etc are also considered in the design of pulse stretcher and compressor.
Though not explicitly shown, optical isolators protect all active stages from backward propagating spontaneous emission and back-reflections. The sequence of light generation and amplification components are entirely fiber optic and fusion spliced together to form a rugged and stable optical path.
3. Experimental result
Figure 2(a) shows the signal average power vs. pump power at the 25 cm Yb3+-doped HEM booster amplifier output and pulse compressor output at 400 kHz pulse rate. The slope efficiency of the booster is 55% with 15 mW input signal power to the booster amplifier. The optical spectra of the input signal as well as the 38 µJ output signal of the HEM booster amplifier are shown in Fig. 2(b) and its inset (logarithmic scale), respectively. As shown in Fig. 2(b), the spectral shape of the output signal at 38 µJ is well maintained through the HEM booster amplification with small amount of spectral modulation caused by SPM.
With 70 W of cladding pump power at 976 nm and 15 mW input signal coupled into the HEM amplifier, the booster output signal reaches 32 W average power and 80 µJ energy per pulse at 400 kHz. The output power doesn’t show any degradation after >100 hours burn-in indicating negligible photodarkening. The measured polarization extinction ration (PER) at 80 µJ and 32W output is 15.4 dB which corresponds to degree of polarization (DOP) of 95%. The measured beam propagation parameter, or M2, at amplifier output of 38 µJ and 30 W is 1.14 (horizontal) and 1.13 (vertical), demonstrating nearly diffraction limited spatial beam quality, as shown in Fig. 4(a). The near field and far field beam profiles at the HEM booster amplifier maximum output are also shown in Fig. 3(a). The slight asymmetry of the beam is likely due to the 8° angled polish of the end-cap.
Figure 3(a) shows the background-free, second harmonic generation (SHG) intensity autocorrelation of the <5 µJ pulses output from the system pulse compressor. The FWHM of the measured autocorrelation trace is 534 fs. From this, a sech2 pulse envelope with 0.65 deconvolution factor was assumed to estimate the duration of the pulse to be 346 fs. Figure 3(b) shows the autocorrelation of 62uJ output energy from the system pulse compressor without compensating SPM and autocorrelation trace. Figure 3(c) is after compensating SPM by adjusting dispersion in the system pulse compressor. When SPM is introduced inside the fiber amplifier as output energy is increased, the pulse broadening is definitely observed in Fig. 3(d) and it is a consistent method to quantify the amount of B-integral in the fiber optic CPA system. When output energy of the system pulse compressor is increased to 62uJ, the pulse got broadened up to 990fs and by adjusting dispersion of pulse compressor the output pulse got compressed down to 380fs.
The M2 values at the compressor output of 25 µJ and 20 W are 1.25 (horizontal) and 1.26 (vertical). Further free space compressor alignment optimization may minimize the aberration and improve the beam quality at the compressor output.
We have for the first time experimentally demonstrated a 1.03 µm monolithic PM fiber-optic CPA system generating 62 µJ per pulse with less than 400 fs pulse duration at a repetition rate of 400 kHz (25 W average power). It is anticipated that improvements, such as further increase in Aeff, increasing Yb3+-doping concentration, or longer pulse stretched pulse will enable pulse energy well in excess of this 62 µJ milestone. Monolithic fiber optic CPA system based on LMA Yb3+-doped HEM fiber amplifier shows a great potential of 1) reliable industrial ultrafast fiber laser owing to the absence of photodarkening and 2) high energy, ultrashort pulse laser system owing to scalability of energy per pulse.
References and links
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