For details please read: H. Okamura, "Shift lens external cavity diode laser for broad wavelength tuning and switching," Optics Letters, Vol. 35, Issue 8, pp. 1175-1177 (2010)

Introduction

A tunable narrow-linewidth laser is an indispensable tool for precision spectroscopy and atom optics. Laser diode alone (LD) offers a continuous scan 10-4 nm due to mode-hop. With the external cavity diode laser (ECDL) , a much narrower line width and a wider scanning range of 10-2 nm can be obtained. There is a need for a light source with a wider scanning range,  and also for wavelength-tunable light sources that are suitable for portable devices,

We proposed an external cavity laser diode that uses an electromagnetically actuated collimating lens for wavelength tuning. Because the wavelength in grating equation is highly sensitive to the incident angle, a slight (~1 µm) transverse displacement of the collimating lens induces a wavelength shift of ~1 nm. We experimentally demonstrate a single-mode, stable, reproducible, and continuous wavelength tuning over a range of 8 nm. The wavelength is proportional to the current applied to the lens actuator, and no hysteresis is observed within the experimental error. By changing the current we achieve fast and reproducible wavelength switching.

Schematic of a Littro-type External Cavity Laser  Diode

The principle

the grating equation,                                      , where θ is the angle of incidence, Φ is the angle of refraction, m is the diffraction order, λ is the wavelength, and d is the grating pitch. For Littrow configuration , θ = Φ and m = 1,

. The wavelength can be tuned by changing the incident angle θ. Instead of tilting the diffraction grating, this can be alternatively achieved by redirecting the incident beam by shifting the collimating lens in front of the LD

Schematic of a a) conventional and b) proposed mechanisms for wavelength tuning.

Advantages

• a slight (~1 µm) transverse displacement of the collimating lens induces a wavelength shift of approximately 1 nm

• pick-up heads (PUHs) for compact disk can be used. low power consumption, leading to portable devices

• Due to small hysteresis of the electromagnetic actuator, good wavelength reproducibility is expected without feedback control.

The wavelength shift

The wavelength shift is obtained by differentiating the grating equation, yielding

Assuming λ = 785 nm, d = 0.83 µm, and a grating of 1200 lines/mm, the required change in the beam angle to shift the wavelength by 1 nm is calculated to be 6.8 × 10−4 rad.

Beam angle change and a lens displacement

the relationship between the beam angle Δθ and the lens displacement s :                      , where f is the effective focal length. For f = 1.2 mm, the lens displacement corresponding to a wavelength shift of 1 nm is calculated to be 0.81 µm, (~ track pitch of a DVD disk)

Evaluation of astigmatism

Shifting the collimating lens causes the lens to move slightly off-axis, which leads to astigmatism and aberrations in the output beam.

a single point ray source which emits rays in a cone like direction of 23° and a 2 mm-diameter plano-convex spherical lens having a focal length of 2 mm. no distortion was observed until lens displacement reached 0.1 mm

Experimental result

Plot of tuned wavelength as a function of the voltage applied to the lens actuator. A beam profile and a scan curve from a Febry-Perot interferometer (finess ~ 25) are shown in the inset. It was found that the wavelength tuning was linearly proportional to the applied voltage to the PUH lens. No hysteresis was observed within the experimental error.

Conclusion

We proposed a ECDL that uses an electromagnetically actuated collimating lens for wavelength tuning.

We demonstrated that by using a commercial PUH lens, a single-mode, stable, reproducible, and continuous wavelength tuning over a range of 8 nm is achieved.

The wavelength shift is proportional to the current applied to the PUH lens, and within the experimental error, no hysteresis was observed.

It is possible to switch to an arbitrary wavelength simply by changing the current applied to the PUH lens.

Useful for applications where fast and reproducible wavelength switching is needed. portable devices due to Simpleness and low power consumption. possible applications : leak detection or explosive detection.