Using diode laser for atomic physics (I) 學生：蔡政佳 指導老師：鄭王曜教授

Outline Introduction Basic laser characteristics Practical guide to using diode laser

Introduction Laser is now the standard tool in atomic physics lab. Why diode laser? i. improving in reliability, power, wave- length coverage, and decreasing in cost. ii. minimization of the system volume. iii. stable amplitude.

Basic laser characteristis The structure of a laser diode

Wavelength is determined by the band gap of the diode. The output power is as a function of the injection current and temperature. IF↗ ，P ↗ T↗，P↘ for fixed IF

A. Beam spatial characteristics Diode laser is NOT Gaussian. i A. Beam spatial characteristics Diode laser is NOT Gaussian!! i. elliptical beam： because of the different divergence angles of the beam(30°×10°), which is emitted from a small rectangular region(0.1μm×0.3μm). solution：anamorphic prisms ii. astigmatism： solution：spatial filtering and lenses

FM spectrum. i. Natural scales to the oscillation spectrum:. a FM spectrum i. Natural scales to the oscillation spectrum: a. The spacing of the cavity modes. b. The linewidth of a single mode. At the present time, it is routine to obtain “single-mode” laser, so the lindwidth relatively matters. But some caveats : -No absolutely single mode laser. -The commercial single mode laser would not run in single mode for all injection currents. (which is always multimode in very low current) ii. The cavity is so short , that the quantum-limited linewidth is significant.(~MHz), even much more smaller than the effect of the environmental factors.

Tuning characteristics. i. Band gap of the semiconductor material. ii Tuning characteristics i. Band gap of the semiconductor material ii. Temperature - optical length - gain curve however, higher temperature will decrease laser’s lifetime, and lower will bring some complications such as water condensation. iii. Injection current - Temperature - Carrier density (index of refraction) Unfortunately, the injection current affects both AM and FM.

Practical guide to using diode laser Mounting of diodes and related optical elements i. Temperature and mechanical stability. ii. As compact as possible. iii. Rigidity. Example: - small lens close to the diode is easier to keep feedback constant but causes more feedback. - microscope objectives absorb infrared wavelength.

Temperature control i. To control the temperature to keep the feedback steady. ii. Second stage of temperature control or more. iii. Low-temperature-coefficients resistors. iv. Enclose the system in a container - keeping dust - isolating from acoustic vibrations Current control i. Low-noise current source ii. Protection against unwanted transients

Tuning to an atomic transition. i Tuning to an atomic transition i. Setting the wanted current roughly and fine-tuning by temperature. ii. Be careful of the optical feedback iii. Using low current to maximize the diode lifetime, if only small power is needed Avoiding unwanted optical feedback i. Diode laser is sensitive to optical feedback - the flat gain curve as a function of wavelength - the cavity finesse(Q) is quite low - the cavity is short ii. Optical isolator : attenuator, Faraday isolator,…

Aging behavior. i. High temperature and high current contribute to Aging behavior i. High temperature and high current contribute to aging. ii. Symptoms of aging - frequency drift - emit more in high longitudinal modes - increasing spectral width

Catastrophic failure modes. i. Switching transients. ii Catastrophic failure modes i. Switching transients ii. Accidental disconnection of the power supply or intermittent connection in power cables iii. Discharges of static electricity iv. Other high voltage arcs in other parts of the lab v. solutions : - Good grounding and shielding procedures - Protective circuits ( which must be as close as possible to the laser)