AdvR (United States)

We investigate efficient frequency-doubling of low energy femtosecond pulses in bulk and waveguide nonlinear crystals, thereby demonstrating how to achieve a compact and portable ultrafast blue lig ...

Entangled photon pairs are created by a system consisting of a 1064 nm pump diode laser that is fiber coupled to a high generation rate photon pair source. The source is a dual element periodically poled potassium titanyl phosphate (KTP) waveguide that up-converts 1064 nm photons to single 532 nm photons in the first stage. In the second stage, the green photons are down converted to energy entangled photon pairs at 800 nm and 1600 nm. The output photon pairs are guided by fiber to sorting optics where they are separated and sent into high-efficiency photon detectors. In particular, the 1600 nm photons are detected by a superconducting nanowire with efficiency over 60% and dead time less than 50 ns. Detector output electrical signals are sent to a time tagger with bin resolution as narrow as 25 ps for coincidence counting. The ultimate goal of this setup is to demonstrate a singlesource, high efficiency, high data rate, quantum communication system to enable Earth-space quantum networks. Of particular interest is a source of entangled photons that is amenable to utilization in aircraft and spacecraft under rigorous flight and environmental conditions. Test results that characterize the entangled photon pair creation and detection capabilities of our system will be presented.

The combination of high brightness laser diodes and periodically poled (PP) waveguide crystals for the generation of blue light at the technically interesting wavelength of 488 nm is promising. Although PPKTP has a lower nonlinear coefficient compared to PPLN it is of interest for the use in such devices. Because of its higher photorefractive damage threshold, it is well suited for operation at room temperature. In this work, a DFB laser as well as a tunable external cavity enhanced broad area diode laser (ECDL) are used for second harmonic generation using a waveguide PPKTP crystal. Both lasers yield several hundred Miliwatts of diffraction limited light around a center wavelength of 976 nm with excellent spectral properties. The ECDL system is further tunable over a broad range of 40 nm. The PPKTP crystal has a length of 12 mm and the 4 μm x 8 μm waveguides are manufactured by ion exchange followed by a patented submount poling technique. By using a DFB laser diode as pump source a laser to waveguide coupling efficiency of more than 55% could be achieved. A maximum output power of 66.7 mW could be generated out of 220 mW infrared light inside the waveguide channel at room temperature. This results in a conversion efficiency of more than 260%/W.

Quasi-phase matched (QPM) frequency conversion in ion exchanged potassium titanyl phosphate (KTP) waveguides can be used for highly efficient single pass conversion of low power cw and quasi-cw lasers. Applications include frequency doubling diode lasers for display and biomedical, pulsed sources for fluorescence and remote sensing, and recently KTP waveguides have been used to generate photon pairs using both Type I and II down conversion for quantum information science and technology (QUIST) applications. In this paper, we will describe a nondestructive, all optical technique that can be used to assess the quality and modal index of the ion exchanged waveguide before periodic poling. The structure of the waveguide is interrogated utilizing Type II sum frequency generation (SFG) and is enabled by the fact that the ion exchange process results in waveguides that can support both TE and TM optical modes. The results of this technique can be used to determine the uniformity of the created waveguide and are used to determine the necessary period for a desired poling result. Furthermore, this technique can be utilized to provide an in situ assessment of the poling for any QPM period without needing the laser sources for the particular frequency conversion interaction. Experimental results will be reviewed.

Diode lasers provide a high degree of flexibility in signal shaping. Picosecond pulses with repetition rates from single shot to 80 MHz or arbitrary modulation formats with GHz bandwidth can be achieved through appropriate electrical drivers without changing the optical configuration. The limitations, however, of single mode diode lasers are low (mW) power levels and a lack of emission wavelengths between 470 and 630 nm. Optical amplification can extend single mode diode lasers to higher power levels where frequency doubling becomes a suitable option e.g. for producing green light at 530 nm. Ytterbium-doped fiber amplifiers (YDFA) show robust and stable operation at 1064 nm with amplifications of about 20 dB. We present a fiber amplified and frequency doubled diode laser that emits green picosecond pulses at variable repetition frequencies with an average output power of several milliwatts. Compared to existing semiconductor-amplified systems, higher stability at significantly smaller size and lower power consumption is achieved.