A double-pass monochromator for wavelength selection in an optical spectrum analyzer - internal construction of the HP 71450A and 71451A optical analyzers - Technical

Hewlett-Packard Journal, Dec, 1993 by Kenneth R. Wildnauer, Zoltan Azary

The wavelength-selection scheme used in the HP 71450A and HP 71451A optical spectrum analyzers propagates the light from the device under test twice through the refraction and diffraction elements in the monochromator.

For many users of spectral analysis instruments, measurement speed is of primary concern, and having a display of an optical spectrum in real time is highly desirable. Many users interested in the purity of their source are also interested in being able to detect low-level signals that are very close in wavelength to the primary signal. The ratio of the power of these low-level signals to the main signal can be easily smaller than 10.sup.-4 (-40 dBc) at offsets less than one nanometer away. The ability of an instrument to resolve or display these signals will be referred to as close-in dynamic range in this article.

Higher transmission rates, better transmission quality, and the longer transmission distances of today's fiber-optic transmission systems have created the need to measure and analyze these low-level optical signals. To measure low-level optical signals an instrument must be efficient and sensitive. To perform these measurements quickly is an added challenge. Also, many times the polarization state (i.e., the orientation of the electric field) of the input signal is either not known or variable. Hence, the instrument measurements should be relatively insensitive to changes of the input polarization state.

Because many applications find it useful to filter an input signal optically, an instrument that can produce optical output should also have variable optical bandwidth. With the need for more precise optical measurements, the instrument should be repeatable and accurate and be able to make these measurements in a standard instrument environment--without the need for an optics table. Finally, it would be convenient if the instrument that measures these low-level signals could be small and rugged enough so that it can be moved around without any special care.

The HP 71450A and 71451A optical spectrum analyzers provide the features mentioned above by using a specially developed wavelength-selection scheme--the double-pass monochromator. A block diagram of these analyzers is shown in Fig. 1. This article describes the operation and performance of the double-pass monochromator and the operation and characteristics of the components in the data acquisition and processing system in Fig. 1.

Double-Pass Monochromator

A double-pass-monochromator-based design was chosen for the HP 71450A and HP 71451A optical spectrum analyzers rather than a spectrometer-based design for two reasons. First, a single photodetector (which is what the monochromator uses) has an inherent advantage over the detector array of the spectrometer for close-in dynamic range measurements. The detector array also costs more than a single detector. Second, a monochromator puts less demand on the optical system for imaging a large span of spatially dispersed wavelengths. The double-pass monochromator configuration further increases the close-in dynamic range of the instrument, essentially obtaining the range of two cascaded monochromators.

A refractive optical system was chosen to reduce the size requirements of the monochromator so as to minimize the overall instrument size. Careful design of the refractive elements has reduced the inherent chromatic aberrations associated with such systems to a tolerable level. The inherent disadvantage of slower measurement speed because of scanning with the diffraction grating is reduced with a direct-drive system and the properties of the double-pass configuration.

Operation. The propagation of light through the system starts with the light entering the monochromator from the device under test (see Fig. 2). This light is then relayed by the input connector assembly ([1] in Fig. 2). The user end of this connector assembly is a fiat-polish physical contact with interchangeable adapters to allow connection to standard fiber interfaces. The monochromator end is an angled interface to air. Both interfaces minimize reflections to the user end. The light then propagates towards the lens and is collimated for illumination of the diffraction grating. The diffraction grating is operated in very nearly a Littrow condition* and can be rotated to the desired wavelength. The light is dispersed by the diffraction grating and returns through the same lens to be reflected by the first plane mirror and imaged onto one of the apertures on the rotatable aperture wheel [2]. By rotating the aperture wheel, different aperture widths (slits) and hence resolution bandwidths can be selected by the user. Once the light enters and leaves the aperture slit the first pass of the double-pass monochromator is effectively complete.

The second pass starts when the light exits the aperture slit and is reflected by the second plane mirror and propagates through an achromatic half-wave plate [3]. The half-wave plate is oriented so that it causes a 90-degree rotation of the s and p polarization components as defined with respect to the lines on the diffraction grating.** The beam is again collimated by the same lens and again illuminates the same diffraction grating. However, because of the orientation of the first and second mirrors with respect to the dispersion direction of the diffraction grating, on the second pass through the system the light is not dispersed any further by the diffraction grating but is collapsed or recombined, creating a filtered replica of the input signal. This recombined beam [4] is then imaged by the lens onto a fiber after reflection from a third plane mirror near the fiber. This fiber, which is called the output fiber in Fig. 2, is a piece of multimode fiber that carries the light to the photodetector for conversion into photocurrent for analysis and display. Another function of this output fiber is to act as a second aperture in the system. As an option, this light can be directed to the front panel of the instrument providing an optical output for the user. In front of this output fiber, there is also a mask wheel that is coaxial with the aperture slit wheel and controlled by the same motor. To measure the dark current of the photodetector, this mask wheel can be rotated such that the output signal is blocked.