[Paper Review] A 1 W injection locked cw titanium:sapphire laser

E. A. Cummings, M. Hicken+1 more|ArXiv.org|Dec 5, 2001

Solid State Laser Technologies23 references3 citations

TL;DR

This paper demonstrates a continuous-wave, single-frequency, 1 W injection-locked Ti:sapphire ring laser at 846 nm, achieved by locking a high-power Ti:sapphire oscillator to a low-power, frequency-stabilized diode laser. The system achieves single-frequency operation with only a few milliwatts of injection power, enabling high-power, tunable, narrow-linewidth output with excellent frequency stability and low noise.

ABSTRACT

We report an injection-locked cw titanium:sapphire ring laser at 846 nm. It produces 1.00 W in a single frequency when pumped with 5.5 W. Single frequency operation requires only a few milliwatts of injected power.

Motivation & Objective

• To develop a high-power, single-frequency Ti:sapphire laser that avoids complex frequency-stabilization components like birefringent filters or optical isolators.
• To address the growing cost and scarcity of high-power single-frequency diode lasers at specific wavelengths by offering a cost-effective alternative using injection locking.
• To demonstrate that injection locking can achieve stable, high-power, single-frequency operation in cw Ti:sapphire lasers, a previously unreported configuration.
• To enable tunable, narrow-linewidth output across the Ti:sapphire gain bandwidth by leveraging commercially available tunable diode lasers as master oscillators.

Proposed method

• A 532 nm pump laser (up to 5.5 W) is focused into a 10 mm long, Brewster-cut Ti:sapphire crystal in a four-mirror folded ring cavity.
• The Ti:sapphire cavity has a 96.6% reflectivity output coupler and mirrors with >99.5% reflectivity, with a 114 mm short arm and 1020 mm long arm to compensate for crystal-induced astigmatism.
• An extended-cavity diode laser (ECDL) at 846 nm, stabilized via Pound-Drever-Hall technique with 37.15 MHz modulation, serves as the master oscillator.
• The master laser output (up to 30 mW) is coupled into a single-mode fiber and mode-matched into the Ti:sapphire cavity with 75% coupling efficiency into the TEM₀₀ mode.
• A piezoelectric transducer on one cavity mirror enables continuous tuning of ~8 GHz, allowing fine wavelength control after injection locking.
• Feedback for locking is achieved via a two-stage integrator circuit using a photodiode to detect a weak reflection from an uncoated quartz flat in the output beam.

Experimental results

Research questions

• RQ1 Can injection locking be successfully applied to achieve single-frequency, high-power operation in a cw Ti:sapphire ring laser?
• RQ2 What is the minimum injection power required to maintain single-frequency operation in such a system?
• RQ3 How does the threshold pump power and slope efficiency of the injection-locked laser compare to the free-running case?
• RQ4 To what extent does the injection-locked laser preserve the spectral purity and frequency stability of the master diode laser?
• RQ5 How does cavity design (e.g., mirror curvature and cavity length) affect the minimum injection power for stable locking?

Key findings

• The injection-locked Ti:sapphire laser achieves a maximum output power of 1.00 W at 846 nm with a slope efficiency of 23%, closely matching theoretical predictions.
• The threshold pump power for injection-locked operation is approximately 1.2 W, lower than the theoretical estimate of 0.3 W, likely due to thermal lensing and beam waist mismatch.
• Single-frequency operation is maintained with as little as 3 mW of injection power at low pump levels, increasing to 15 mW at full output power.
• The optical spectrum of the amplified laser, measured with a scanning Fabry-Perot (FSR = 2 GHz, finesse = 400), shows no significant sidebands, indicating high spectral purity.
• The power spectrum of the relative frequency between the Fabry-Perot cavity and the Ti:sapphire laser is indistinguishable from that of the master laser, confirming excellent frequency stability.
• A smaller cavity (360 mm round-trip length, 50 mm radius of curvature) reduces the minimum injection power by a factor of 3 compared to the larger cavity, indicating improved locking efficiency.