Optical Coherence Topography with Tunable Cavity Surface Emitting Laser

Tu Xiang Zheng

  

US Patent 6,602,427 issued to the present author describes a micromachined optical mechanical modulator based WDM transmitter/receiver module. The Fabry-Perot cavity of the mechanical modulator is structured from a three-polysilicon-layer stack formed on the surface of a single crystalline silicon substrate. The polysilicon membrane and its supporting polysilicon beams of the cavity are cut from the top polysilicon layer of the stack and are released by selective etching of their underlying polysilicon. The etched underlying polysilicon layer is heavily doped and then converted into porous polysilicon by anodization in HF solution. The polysilicon membrane and its supporting polysilicon are finally released using a reactive ion etch process to avoid stiction often generated in a wet etch process. A conic hole is formed on the backside of the single crystalline silicon substrate for receiving an optical fiber that can be passively aligned with the Fabry-Perot cavity.

Optical coherence tomography (OCT) is a non-invasive imaging test that uses light waves to take cross-section pictures of your retina, the light-sensitive tissue lining the back of the eye. With OCT, each of the retina’s distinctive layers can be seen, allowing your ophthalmologist to map and measure their thickness. These measurements help with diagnosis and provide treatment guidance for glaucoma and retinal diseases, such as age-related macular degeneration and diabetic eye disease. OCT can also be used for intravascular imaging of plaque to assess heart disease, cancer biopsy imaging, developmental biology research, art preservation, and industrial inspection.

As shown in the above figure, a called swept-source OCT uses a wavelength-swept laser light source, that is, one whose emission sweeps back and forth across a range of wavelengths. A detector and a high speed analog-to-digital (A/D) converter complete the imaging system. The OCT has several fundamental advantages including ultrahigh imaging speeds, deep tissue penetration, Doppler OCT flow analysis, and long imaging range. With such a compact, high-performance, low-cost swept source for OCT it is possible to achieve a combination of ultrahigh sweep speeds, wide spectral tuning range, adjustability in sweep trajectory, and extremely long coherence length.

Wavelength tuning of the micromachined cavity is accomplished by applying a voltage between the top membrane and bottom membrane, across the air gap. A reverse bias voltage is used to provide the electrostatic force, which attracts the top membrane downward to the bottom membrane and shortens the air gap, thus tuning the laser wavelength toward a shorter wavelength (blue shift). It has been shown that the cavity using electrostatic force follows a 1/3 gap size rule. As the voltage is applied, the top membrane is attracted downwards with a displacement approximately equaling to 1/3 gap size. As increases further, the attractive force cannot be balanced by the mechanical spring force, and the membrane collapse onto the bottom membrane. Increasing voltage further at this point results either no movement or capacitor discharge. The top membrane can be brought back to its original position when the voltage is removed if an appropriate mechanical design is used.

The incident light to the micromachined cavity is emitted by a vertical cavity surface emitting laser. The micromachined cavity transmits a narrow band of wavelengths and rejects wavelengths outside of that band. The cavity will resonate when the following condition is met:

nd cosθ = mλ/2                         (1)

where θ is the incident light angle normal to the mirror, λ is wavelength, d is the micromachined cavity length, n is the refractive index of the medium, and m is the fringe order number. For normal incident light, with air as the medium (n = 1), the resonating micromachined cavity equals multiples of a half wavelength.

By driving the micromachined cavity with specially shaped voltage, the wavelength can be swept in time as required for swept source OCT. In classical physics, where the speeds of the top membrane of the micromachined cavity relative to the bottom membrane are lower than the velocity of laser light, the relationship between observed micromachined cavity transmitted light frequency f and the incident light frequency f₀ is expressed as

f = [(c+υ_r)/(c + υ_s)] *f₀                     (2)

  

Where c is the velocity of light, υ_r is the velocity of the top membrane relative to bottom membrane or air and υ_s is the velocity of the incident light relative to air. It can be seem that the transmitted light frequency or wavelength is decreased if two membranes of the cavity is moving away from the other.

It has been reported that the micromachined cavity can be move very fast, allowing the micron-scale cavity length to be tuned rapidly. It has demonstrated a fundamental repetition rate of 600 kHz, which for OCT purposes allows its individual scans to be acquired at rates as high as 1.2 MHz through the use of both forwards and backwards sweeps.