12 Oct 2007
Fabricating microlenses directly on to the top of a VCSEL's gain structure creates a versatile single-mode source.
A tunable single-mode VCSEL that uses an intracavity polymer microlens could be ideal for applications such as spectroscopy and metrology, say researchers at Strathclyde University, UK. The simple idea can also be extended to work with VCSEL structures emitting at any wavelength. (Optics Letters 32 2831)
"We wanted to develop a device with an 'open access' cavity into which analytes, gases and even cold atoms could be inserted," Nicolas Laurand from Strathclyde's Institute of Photonics told optics.org. "This is the first time to our knowledge that a microlens has been used as an intracavity element in a tunable VCSEL."
The team's fiber-VCSEL comprises a bottom semiconductor distributed Bragg reflector followed by the active region. The polymer microlenses are fabricated directly on top of the active region. There is then a small air gap before an optical fiber with a dielectric mirror at its tip completes the cavity.
According to Laurand, this configuration has many significant advantages. "The output power is inherently fiber-coupled with the fiber mirror acting as the output coupler," he explained. "The microlens acts as a mode discriminator and maximizes the overlap between the pump spot and the fundamental cavity mode. This allows power scaling while keeping single transverse mode operation. The microlens also allows a flat fiber mirror to be used, which optimizes coupling of the intracavity power into the fiber mode."
This approach can also be employed for VCSEL structures emitting at any wavelength. "This configuration is versatile because different wavelength windows can be reached simply by changing the gain structure and, if necessary, the optical fiber," commented Laurand.
The team's main challenge was to fabricate the microlenses directly on top of the semiconductor gain structure. Once this had been perfected using a resist-reflow technique followed by reactive ion etching, the next step was to investigate the emission and tunability characteristics.
"The fiber is fixed on a piezoelectric translator which allows us to fine tune the position of the fiber-mirror with respect to the gain structure," said Laurand. "This in turn changes the cavity length and the oscillating wavelength. The best overall laser characteristics are obtained for a gap of approximately 30 microns."
The researchers found that their VCSEL was continuously tunable between 1028 and 1041 nm. For a pump power of 60 mW from an 808 nm diode laser, they report that the VCSEL's output power remained above 6.5 mW across the entire tuning range. "We use a standard commercially available single-mode fiber with a mode field diameter of 8 microns at 1000 nm," added Laurand.
Laurand says that the team is working on further power-scaling of its fiber-VCSEL and hopes to obtain an output power of tens of milliwatts. "We also plan to characterize the linewidth and noise properties, which are important to gauge spectroscopy capabilities."
Jacqueline Hewett is editor of Optics & Laser Europe magazine.