Optical Spectrum Analyzer Monochromators: Single-Pass vs. Double-Pass Architectures
Filed under Fiber Optics Comm
This guide outlines the core differences between single-pass and double-pass monochromator designs in diffraction-grating-based optical spectrum analyzers (OSAs). In fiber optic communications, choosing the correct monochromator architecture is critical for demanding spectral measurements, such as dense wavelength division multiplexing (DWDM) laser characterization and narrowband DFB source testing. This article explains how the internal optical path affects stray light rejection, close-in dynamic range, and overall measurement sensitivity, helping test engineers select the appropriate instrument for their optical validation tasks.
- Keysight 86146B Monochromator
- Patented double-pass design
- Anritsu MS9710C Dynamic Range
- 70 dB at +/- 1.0 nm
- Keysight 86146B Max Input Power
- +30 dBm
- Keysight 86140B Res Bandwidth
- 0.06 nm to 10 nm
Optical Paths of Single-Pass and Double-Pass Monochromators
In diffraction-grating-based optical spectrum analyzers, the monochromator separates incoming optical signals into constituent wavelengths. In a single-pass design, light passes through the grating once before hitting the photodetector. While simple, single-pass monochromators can suffer from higher levels of stray light because unwanted wavelengths are scattered within the cavity rather than fully filtered out.
By contrast, a double-pass design reroutes the light so that it passes across the dispersive element a second time before reaching the detector. This secondary pass significantly sharpens the spectral selectivity of the instrument and reduces the level of stray light that reaches the detector, allowing for cleaner separation of closely spaced wavelengths.
Stray Light Rejection and Dynamic Range
Close-in dynamic range is a key performance metric when characterizing DWDM systems or narrowband laser sources. Stray light in a single-pass monochromator raises the instrument's effective noise floor close to the peak carrier signal, masking adjacent low-level signals.
Instruments employing a double-pass monochromator, such as the Keysight Technologies (Agilent HP) 86146B, achieve high close-in dynamic range. The Keysight Technologies (Agilent HP) 86146B provides a dynamic range of >= 55 dB (with chop off) or >= 60 dB (with chop on) at an offset of +/- 0.5 nm, rising to >= 70 dB (with chop on) at +/- 1.0 nm. For comparison, the Anritsu MS9710C, which utilizes a high-performance diffraction grating, achieves a dynamic range of 42 dB at +/- 0.2 nm and 70 dB at +/- 1.0 nm.
Resolution and Input Power Limits
Selecting the correct instrument also involves balancing resolution bandwidth (RBW) options and input power limits.
- The Keysight Technologies (Agilent HP) 86140B offers resolution bandwidth settings from 0.06 nm to 10 nm, with a minimum noise floor of -80 dBm and a maximum safe input power of +20 dBm.
- The Keysight Technologies (Agilent HP) 86146B supports a resolution bandwidth of 60 pm to 10 nm, a power level sensitivity down to -90 dBm, and a higher maximum safe input power of +30 dBm.
- The Anritsu MS9710C provides a resolution setting range of 0.05, 0.07, 0.1, 0.2, 0.5, and 1 nm, with an optical reception sensitivity of -90 dBm and a safe maximum input power of +23 dBm.
Example instruments
Frequently asked questions
- What is the primary benefit of a double-pass monochromator in an OSA?
- A double-pass monochromator, like the patented design in the Keysight Technologies (Agilent HP) 86146B, passes the optical signal across the diffraction grating twice. This significantly reduces stray light and improves close-in dynamic range, allowing engineers to resolve weak signals located very close to strong peak carriers.
- How do chopper settings affect OSA dynamic range?
- In instruments like the Keysight Technologies (Agilent HP) 86146B, turning the chopper on improves the dynamic range. For example, at +/- 0.5 nm offset, the dynamic range improves from >= 55 dB (chop off) to >= 60 dB (chop on), and at +/- 1.0 nm, it improves from >= 60 dB (chop off) to >= 70 dB (chop on).