We report on the development of near-infrared high dispersive mirrors (HDM) with a group delay dispersion (GDD) of −2000 fs2. A HDM pair based on one optimized result at two reference wavelengths (1550 nm and 1560 nm) can reduce the total oscillation of the GDD effectively in the wavelength range of 1530-1575 nm. This HDM pair is designed and fabricated in a single coating run by means of the nonuniformity in film deposition. For the first time, near-infrared HDMs with two different reference wavelengths have been successfully applied in an erbium-doped fiber chirped pulse amplification system for the compression of 4.73 ps laser pulses to 380 fs.
© 2016 Optical Society of America
The field of ultrafast lasers has fascinated many researchers over the last decades for their potential applications in attosecond pulse generation , industrial applications  and medical applications . Dispersive mirrors (DM), which provide precise control of group delay dispersion (GDD), have been a key element in these ultrafast laser systems [4–9]. High dispersive mirrors (HDM) with large dispersion compensation have become one of the main development directions of DM in the past ten years. Presently, the majority of femtosecond laser systems, such as chirped pulse amplification (CPA) [10,11] and femtosecond laser oscillators [12–15], include the HDM optics due to their characteristics of low- losses and high- dispersion [16,17].
Fiber-based CPA lasers offer high integration level, which results in an unprecedented robustness and compactness suitable for commercial applications . However, conventional fiber CPA systems depend on rather lossy, complex and alignment-sensitive systems of gratings  or fibers  for pulse stretching and compression, which compromise the production efficiency and spatio-temporal quality of the amplified pulses. When diffraction gratings are used in stretchers/compressors, the relatively low diffraction efficiency limits the output power of CPA systems. Meanwhile, the alignment sensitivity of these components poses a serious challenge in the day-to-day operation of CPA systems. For fiber compressors, because of the inevitable nonlinear effects, they may result in pulse fracturing or introduce a pulse pedestal occupying large amount of the pulse energy, which obviously reduces the pulse quality.
HDM compressor-based CPA systems have shown excellent results in Ti:sapphire laser system [10,11]. CPA systems with HDM compressors offer a remarkable simplification of ultra-short pulse lasers and afford potential for their advancement to shorter pulse durations, higher peak powers, and higher average powers with user-friendly systems . Furthermore, CPA systems implemented with HDMs are intrinsically free from the angular chirp and nonlinear effects because of their low losses, high dispersion and precise dispersion control.
In this study, we report near-infrared HDMs with a GDD of −2000 fs2 for an erbium (Er) fiber CPA system. In order to reduce the total GDD oscillations of HDMs required in the CPA, a HDM pair based on one optimized result used at two reference wavelengths is designed and fabricated in a single coating run by the nonuniformity in deposition. As a first application of HDMs in the Er fiber CPA system, we utilize them to compress 4.73 ps laser pulses to approximately 380 fs with a total efficiency of more than 90%. Such near-infrared HDMs may open new horizons in the application of fiber CPA systems.
2. Design of HDMs
HDMs are able to compensate thousands of femtoseconds squared with just several percent of total losses in pulse compressors with ten or more bounces. However, the design of such HDMs can be challenging. The commercial OptiLayer software , with powerful needle optimization  and gradual evolution algorithms , was used to design the HDMs. Here, near-infrared HDMs with a GDD of about −2000 fs2 were demonstrated.
Tantalic oxide (Ta2O5) and silicon dioxide (SiO2) were chosen as the layer materials. The refractive indexes of both layer materials, specified by the Cauchy formula, are depicted in Fig. 1.
Considering the large amount of dispersion compensation required in the CPA system and the relatively small single reflection dispersion of the HDM, many bounces between two HDMs should occur. Therefore, a small incident angle (10°) was chosen to design HDMs. However, the increase in bounces may lead to the aggravation of GDD oscillations, which may affect the quality of laser pulses compressed by the HDMs. Thus, a HDM pair with two different reference wavelengths was designed to decrease the total GDD oscillations. At first, the reference wavelength was chosen as 1550 nm. The target design was optimized to have a GDD of −2000 fs2 and a reflectance of 100% in the wavelength range of 1520-1580 nm. A gradual evolution algorithm was utilized to optimize the layer thickness. The theoretical GDD and reflectance of the optimized HDM are shown in Fig. 2 (red line). Based on the optimized result, when the reference wavelength was changed to 1560 nm, antiphase oscillation of the GDD was achieved, as shown by blue line in Fig. 2. The effective reflectance of HDMs with two reference wavelengths is the geometric mean of reflectances for these two reference wavelengths and the effective GDD is the arithmetic mean of GDDs for these two reference wavelengths. The effective GDD (green line in Fig. 2) exhibits a significant reduction of the GDD oscillation, especially in the wavelength range of 1530-1575 nm. This HDM pair draws on one optimized result used at two reference wavelengths. Antiphase oscillations of the GDD are obtained from a property of multilayers: the reflectance and GDD become red-shifted with an increase of the reference wavelength.
The concept of this HDM pair design is based on the nonuniformity in film deposition. This nonuniformity, which influences the deposition thickness of the layers, may result in a shift of practical reference wavelength. Furthermore, the practical reference wavelength is related to the distance from the substrate to center of the fixture plate. Therefore, by adjusting the distance between substrates to control the nonuniformity in deposition, HDMs with reference wavelengths of 1550 nm and 1560 nm can be obtained at different positions in a single coating run.
3. Fabrication of HDMs
Successful manufacture of HDMs can be challenging because of the high sensitivity of GDD to even small errors of layer thicknesses. The magnetron sputtering [6–8] and ion-beam sputtering  have proved to be the most reliable techniques for depositing complex dielectric films during the past decades. Our designed HDMs were deposited by the dual-ion-beam sputtering technique with a planetary rotation system. The total nonuniformity of this system was about 1%, and the two substrates were placed at suitable distances from the plate to control the thickness nonuniformity to be 0.6%, which corresponded to a 10 nm shift of reference wavelength. Therefore, HDMs with reference wavelengths of 1550 nm and 1560 nm were fabricated in a single deposition run.
After fabrication, the transmittance spectra were measured using a PerkinElmer spectrophotometer (Lambda-1050), as shown in Fig. 3. The practical reference wavelengths of the fabricated HDMs in a single coating run, which are inversed by the transmittance spectra, are 1550 nm and 1560 nm.
The GDD values of HDM with reference wavelength of 1550 nm were determined by a home-built white light interferometer [24,25]. Figure 4 summarizes the comparison of the theoretical and measured data. Taking into consideration the high sensitivity of the GDD to deposition errors, the agreement between theory and experiment appears to be satisfactory.
4. HDMs in Er fiber CPA system
To test the utility of our near-infrared HDMs, we apply them to an Er fiber CPA system for compressing pulses. The schematic layout of the experimental setup is shown in Fig. 5. The seed pulses are delivered by the Er-doped fiber laser oscillator based on nonlinear polarization rotation technology (NPR). They are stretched by the dispersion compensation fiber (DCF) with positive dispersion of 150 ps/nm/km, which is spliced before the amplifier. The stretched pulses are amplified by an Er gain fiber, which is reversely pumped by a laser diode (LD). The pulse duration after amplification is 4.73 ps (full width at half maximum, FWHM) measured by an autocorrelator.
In the compressor, instead of conventional gratings or fibers, our near-infrared HDMs with a GDD of about −2000 fs2 were employed to compress the pulses. HDMs with reference wavelengths of 1550 nm and 1560 nm were considered as a HDM pair. A total of four HDMs for an angle of incidence of approximately 10° were utilized to compensate the GDD of −188000 fs2 with 94 reflections. The compressed pulses were characterized by the autocorrelator. Assuming a Gaussian pulse shape, the pulse duration is at a level of 380 fs (FWHM), as shown in Fig. 6. In this CPA system with the HDM compressor, the overall throughput is more than 90%. Moreover, there is no pedestal in autocorrelation trace, demonstrating that the HDM compressor does not introduce nonlinear effects or high order chromatic dispersion.
The HDM compressor has successfully compressed the 4.73 ps pulses to 380 fs. The advantages of HDM compressor in the Er fiber CPA system are the alignment insensitivity and free from nonlinear effects, which reduce the difficulty of system adjusting efficiently and support stable day-to-day operation of CPA system.
Near-infrared HDMs with a GDD of −2000 fs2 have been demonstrated. Based on the nonuniformity in deposition, a HDM pair with different reference wavelengths has been designed and fabricated in a single coating run, being able to suppress undesirable GDD oscillations especially in the wavelength range of 1530-1575 nm. The advantage of this approach is an increased stability against deposition errors compared to the conventional complementary-pair approach. Furthermore, the new method requires only one coating run for producing all HDMs in contrast to the conventional route relying on two perfectly-matched coating runs, which dramatically reduces of deposition costs and deposition time. Our produced HDMs, which compressed 4.73 ps pulses to 380 fs with low overall losses (<10%), are successfully used in the Er fiber laser CPA for the first time.
Employing the HDM compressor will benefit the development of stable, high efficient and high pulse quality Er CPA systems and may even open the prospect for pulse stretching and compression in other fiber CPA systems.
National Natural Science Foundation of China (NSFC) (61308021); the Innovation Fund of Research Center of Laser Fusion (2015-05).
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