Diode-pumped solid state lasers and intracavity frequency conversion

Introduction: Nd³⁺ was the first of the trivalent rare earth ions to be used in a laser, and it remains by far the most important element in this group. Stimulated emission has been obtained with this ion incorporated in at least 100 different host materials, and a higher power level has been obtained from Nd lasers than from any other four-level material.  In our work we focus on the principal host materials which are of importance for diode pumping like YAG and vanadate crystals. In these hosts, stimulated emission is obtained at a number of frequencies within three different groups of transitions centered at 0.9, 1.06, and 1.3 μm. Radiation at these wavelengths results from ⁴F_3/2→⁴I_9/2, ⁴I_11/2, ⁴I_13/2 transitions, respectively as depicted in fig.1. For all these transitions ⁴F_3/2 serves as the upper laser level and ⁴I_9/2 serves as the ground level. Above the upper laser level are located the pump bands starting with the manifold ⁴F_5/2, which is

Fig1. Energy level diagram of Nd³⁺ doped crystal                                Fig2. Intracavity wavelength conversion

responsible for absorption around 808 nm. Intracavity frequency doubling of these wavelengths not only efficiently generates radiation in the important  red, green and blue spectral regions but also other useful visible radiations can be obtained by intracavity frequency mixing of these wavelengths as shown in fig.2. High power light sources in the visible region are of great importance for a number of scientific and technical applications such as in industry, metrology, remote sensing, medicine and even in entertainment.

Diode-pumped solid-state lasers (DPSSL) can be broadly divided into two categories namely diode-end-pumped system and diode-side-pumped system. Diode-end-pumped system provides high efficiency and good beam quality of the laser due to excellent geometric match between the pump beam and the cavity mode but has the disadvantage in power scaling due to the stress induced fracture of the laser crystal. The side-pumped configuration is useful for power scaling purpose because in this configuration longer laser crystal rod can be used and more number of diode lasers can be accommodated by placing them side-by-side. In SSLD, we have studied experimentally and theoretically the performance of DPSSL under side and end-pumping geometry in CW and pulsed configuration at the fundamental and intracavity generated second harmonic wavelength. A summary of the recent activities are listed below:

• High power Diode side-pumped intracavity frequency doubled Nd:YAG laser:

High power green beam generation at 532 nm

• High power continuous wave (CW) green beam generation: Around 82 W of stable CW green beam at 532 nm is obtained by intracavity frequency doubling of diode side pumped Nd:YAG/KTP laser in a Z-shaped resonator at a total pumping power of  756 W corresponding to more than 11% optical to optical conversion efficiency.
• High average power green beam generation: More than 145W of average green beam at 532 nm is demonstrated in a symmetric linear cavity configuration by intracavity frequency doubling of AO Q-switched Nd:YAG laser. A type-II phase matched LBO crystal was used as a nonlinear crystal for second harmonic generation. A maximum 146W of average green power with 90 ns of pulse duration is obtained at a repetition rate of 30kHz corresponding to more than 19% of optical to optical conversion efficiency. The beam quality factor (M²) of the green beam was measured to be ~30.
• High average power green beam with short pulse duration: A novel coupled cavity configuration is demonstrated to reduce the green pulse width while maintaining the high average power. In an Nd:YAG/KTP based green laser system more than 80 W of average green power with less than 50 ns green pulse duration is demonstrated by employing this coupled cavity configuration at 16 kHz repetition rate.

2.  Diode end-pumped solid state lasers: Multicolor laser beam generation:

       Red beam at 671 nm               Blue beam at 457 nm               Yellow beam at 593.5 nm

• Green laser: More than 5 W of CW green beam at 532 nm is obtained from Nd:YVO₄/KTP based compact laser system under diode- end-pumping configuration corresponding to ~ 30% optical to optical conversion efficiency in a nearly diffraction limited beam quality ( M² <1.2). The work is culminated in the development of ophthalmic laser system.
• Operation at 1342 nm and high power Red beam generation: 7 W of CW IR power at 1342 nm and  2 W of CW red beam at 671nm is obtained by exploiting the ⁴F_3/2→⁴I_13/2 laser transition in  Nd:GdVO­­­­₄ or Nd:YVO₄ crystal and by its intracavity frequency doubling.
• Quasi-three level operation and Blue beam generation: The quasi-three level ⁴F_3/2→⁴I_9/2 transition in Nd:YVO₄ crystal is exploited to generate  2.3W of CW output at 914nm and  1W of blue radiation at 457nm by its intracavity frequency doubling. Lasing operation at 946 nm and blue beam at 473 nm also demonstrated in Nd:YAG laser from the quasi-three level laser transition.
• Yellow beam generation: CW Yellow beam at 593.5 nm is generated by intracavity sum frequency mixing of 1342nm & 1064nm in a dual laser coupled cavity configuration.

3.    System development:

• Green laser photocoagulator for the treatment of diabetic retinopathy: The activities on diode-end-pumped green lasers carried out at the lab are culminated in the development of a “laser photocoagulator”. The green laser photocoagulator is a medical equipment which is used for the treatment of diabetic retinopathy.  Green laser beam can reach the eye without much absorption in the intermediate ocular media and get absorbed at the excess blood vessels in the retina of the patients suffering from prolonged diabetes enabling controlled cutting and sealing of these blood vessels. The system developed at SSLD, RRCAT is handed over to ‘Aravindo Eye Care Hospital’, Madurai in August, 2008 and is being successfully used for the treatment of diabetic retinopathy. A photograph of the system and a treated retina with this system are shown below.

       Green laser Photocoagulator developed at RRCAT               A portion of treated retina with this system

• RGYB laser system: The studies on different laser transitions in Nd-doped laser crystals are culminated in the development of a compact multicolor laser system simultaneously producing green, red, blue and yellow beam in a nearly diffraction limited beam quality. The individual color are useful for various scientific, spectroscopic and medical application and all the colors simultaneously can be used for laser projection or laser show.

Multicolor ( RGYB) laser system

4. Other activities:

• QML operation using nonlinear mirror saturable absorber producing 40 microJ picoseconds pulse
• Simultaneously Q-switched and mode-locked intracavity frequency doubled laser using Cr:YAG
• Dual wavelength operation for THz frequency generation in a hybrid cavity configuration
• Q-switched pulse shape manipulation in a combined end and side-pumped laser system
• Laser operation with other gain media like Nd:CNGG, Nd:KGW etc.
• Green laser pumped OPO in near degenerate configuration.
• Modeling of all the laser systems and cavity configurations: Thermal lens focal length, Fractional thermal loading, thermal birefringence, thermo-optical coefficient (dn/dT -parameter), Effective stimulated emission cross section of the gain medium, M²-parameter.