Directly Chirped Laser Source for Chirped Pulse Amplification

Transcription

1 Directly Chirped Laser Source for Chirped Pulse Amplification Input pulse (single frequency) AWG RF amp Output pulse (chirped) Phase modulator Normalized spectral intensity (db) Wavelength (nm) R. Xin and J. D. Zuegel University of Rochester Laboratory for Laser Energetics Advanced Solid-State Photonics San Diego, CA 31 January 3 February 2010

2 Summary A novel directly chirped laser source (DCLS) shows promise for seeding chirped-pulse amplification systems Chirped-pulse amplification (CPA) systems would benefit from seed sources with improved flexibility and control. DCLS is an all-fiber, chirped-pulse seed laser that provides programmable control of chirped-pulse parameters, and diffraction-limited beam performance expected from single-mode fiber. A prototype DCLS system produced a bandwidth of 1.15 nm centered around 1053 nm that supports a 2.1-ps transform-limited pulse. Temporal interferometry shows that DCLS output pulses are linearly chirped. E18595

3 Conventional CPA systems use a mode-locked laser and grating-pulse stretcher to produce chirped pulses Short-pulse oscillator Grating-pulse stretcher Replace with DCLS Amplifier Pump Grating-pulse compressor E18596 The conventional approach to producing chirped seed pulses has several limitations: Limited flexibility to control dispersion without system realignment Adjustable compressed pulse width is needed for a range of experiments. This requires adjusting grating separation which changes system alignmnet and timing High order dispersion compensation by tilting the grating affects system alignment Mode-locked oscillator and stretcher optics are expensive and have large footprints (especially for large stretch ratios)

4 A directly chirped laser source (DCLS) can be realized by phase modulating a shaped laser pulse m 0 = 1053 nm m 0 = 1053 nm z (t) Narrowband shaped laser pulse After phase modulation Dm 10 nm Df 2.7 THz 2.5 ns t t A quadratic time-dependent phase produces a linear frequency chirp. Changing the final compressed pulse width requires adjusting only the amount of chirp applied to the laser pulse without changing system alignment or timing. High-order phase terms (t 3, t 4, ) can compensate system dispersion and intensity-dependent phase errors. E18597

5 A practical DCLS system depends on a few key available technologies AWG RF amp Input pulse (single frequency) z^th = N Phase modulator Phase modulator Output pulse (chirped) pass r V r V ^ t h 2.5 ns Arbitrary Waveform Generator programmable control of phase modulation: external synchronization with low jitter, RF amplification increases single-pass phase modulation Low-V r phase modulator efficient phase modulation minimizes the number of round-trips in ring cavity. Multipass phase modulator phase-modulation depth adds on successive passes, if properly timed: match cavity length to reference frequency for synchronization, fiber device to simplify multipass architecture no alignment! E18598

6 A DCLS system requires control of the electrical waveform applied to the phase modulator Tektronix AWG710B Arbitrary waveform generator (AWG): sample rates up to 4.2-GSamples/s 2-V peak-to-peak output signal into 50 X with 8 bits of resolution synchronous operation mode (external clock) Voltage (normalized) E AWG output Quadratic fit 1 AWG output amplified Quadratic fit Time (ns) DCLS seed pulse (7.7 k traces) v jitter = 7.4 ps (rms) AWG marker pulse 100 ps

7 A low V r phase modulator (V r = 1 V) provides efficient phase modulation for DCLS 50/50 splitter EO space, LiNO 3 phase modulator V r = 0.9 V at 50 MHz, V r = 1.1 V at 400 MHz Coupler Fiber laser source (cw) Phase modulator Photodiode Scope The phase modulator is built onto one arm of a Mach Zehnder interferometer A sinusoidal signal is applied to the phase modulator Homodyne signal is detected Nonlinear fit is employed to determine V r E18600 V^th IH ^th = I0 > 1+ cos fr p V H r PM signal (normalized) Homodyne signal (normalized) Time (ns) Homodyne signal Nonlinear fit 1 0

8 A prototype DCLS system composed of all-fiber components was built in a rack-mounted chassis 38-MHz clock PLL 10.5-kHz trigger IFES* 2.5-ns optical pulse Bias for switch AWG Amplifier Gate for switch 2 2 optical switch RF amplifier Phase modulator Quadratic driving signal Yb-doped fiber amplifier Variable optical delay line AOM Output E18601 *J. R. Marciante and J. D. Zuegel, Appl. Opt. 45, 6798 (2006).

9 The measured spectrum after 18 round-trips has a width of 1.15 nm and supports a 2.1-ps pulse Normalized spectral intensity (db) nm With phase modulation Pulse intensity (normalized) ps Wavelength (nm) Time (ps) A 2.5-ns pulse acquires a 1.15-nm bandwidth after 18 round-trips. The shape of the spectrum originates from distortion to the quadratic PM signal. The spectrum supports a 2.1-ps Fourier transformlimited pulse (simulation). E18602

10 Temporal interference is employed to characterize the linear chirp x Fast photo diode Intensity (arbitrary units) E Measured homodyne signal 1 Time (ns) Mirror mounted on translation stage * Number of fringes * 11 Data 10 Linear fit z^th = nt 2 I ^th = I 61 + cos ^nxth@ H Translation stage position (mm) *C. Dorrer, Opt. Lett. 29, 204 (2004).

11 Summary/Conclusions A novel directly chirped laser source (DCLS) shows promise for seeding chirped-pulse amplification systems Chirped-pulse amplification (CPA) systems would benefit from seed sources with improved flexibility and control. DCLS is an all-fiber, chirped-pulse seed laser that provides programmable control of chirped-pulse parameters, and diffraction-limited beam performance expected from single-mode fiber. A prototype DCLS system produced a bandwidth of 1.15 nm centered around 1053 nm that supports a 2.1-ps transform-limited pulse. Temporal interferometry shows that DCLS output pulses are linearly chirped. E18595