Frequency-domain Optical Delay Lines

The frequency-domain optical delay line (FD-ODL) is a device used to create an optical delay in an interferometer, and allow the amount of delay to be scanned at a high speed. It can also perform first order dispersion compensation. The FD-ODL has also been called the rapid-scanning optical delay line (RSOD) in research papers on optical coherence tomography.

Brief history

The FD-ODL originated from optical pulse compression techniques (Treacy, 1967, and Martinez, 1984), and was first proposed specifically for group delay generation in pulse autocorrelators (Kwong, 1993). It was applied to optical coherence tomography firstly using a single-pass design (Tearney, 1997) and then a double-pass design (Rollins, 1998) which doubled the achievable delay and made alignment easier.

Method of operation

The figure below illustrates the mechanism by which the FD-ODL generates an optical delay. The light input to the delay line via the collimator impinges on the grating and is dispersed. The dispersed light is then focussed by a lens onto a galvanometer mirror. The reflected beam then passes through the lens, onto the grating, and then strikes a "double-pass" mirror, from where it travels the reverse path through the delay line back to the input. The accumulated delay is varied by rotation of the galvanometer mirror.

FD-ODL delay scanning Tilting the galvanometer mirror changes the optical delay of the FD-ODL

The analysis of the wavelength-dependent phase delay imparted to the beam shows that, for small galvanometer tilt angles, the group delay is linearly proportional to the galvanometer angle. The derivative of phase delay with respect to tilt angle is linearly proportional to the offset of the galvanometer pivot from the lens axis (the direction of offset is in the lens focal plane and perpendicular to the galvanometer axis).

Effectively, a Doppler frequency shift of controllable magnitude and sign is applied to the light by the delay line when it is scanning. This has the advantage that the frequency of the interference fringes can be freely chosen and the detection bandwidth can be set to a frequency range where the electronic detection circuit has a low noise level. The fringe frequency is usually chosen to be high enough that the frequency content of the envelope and fringes are well separated, allowing efficient envelope detection. (Zvyagin 2003)

In the configuration shown below, the input beam is vertically displaced so that it passes above the double-pass mirror.

Out-of-plane FD-ODL configuration In this configuration the collimator is raised so that the input beam and beam reflecting to the double-pass mirror are not co-incident.

In another possible configuration, shown below, the beam is kept in the central plane of the lens throughout the delay line by use of a polarization-mutliplexing scheme (Silva, 1999).

PBS-based design of the FD-ODL The polarizing beamsplitter is used with quarterwave plate to allow the beam to be deflected to the double-pass mirror or pass straight through as required. The input beam polarization should be set appropriately so that the first pass through the PBS is not deflected.

Achieving long scan range

FD-ODLs are typically used in time-domain optical coherence tomography systems. Typically the OCT depth range is 2-3 mm, limited by multiple scattering in biological tissue and, therefore, the FD-ODL does not require a scan range longer than this. However, for a particular application in OBEL (anatomical optical coherence tomography) we have investigated the use of the FD-ODL to achieve a long delay (20-30 mm).

When using an FD-ODL over the largest possible delay range, care must be taken to achieve a high coupling efficiency over the entire range. We have used ZEMAX to model the behaviour of the FD-ODL in terms of spectral throughput and total coupling efficiency. The modelling helped elucidate which factors are important to achieve maximum scan range. For example, it was found it is necessary to keep the galvanometer mirror pivot positioned at the centre of the beam.

FD-ODL modelling Zemax modelling was used to predict the FDODL efficiency for different wavelengths and optical delays, using accurate lens and grating models.

Spectral throughput A measurement of the spectral throughput of a FD-ODL, showing the good result that very little spectral filtering occurs.