Optical spectrum analyzer and method

Abstract: An optical spectrum analyzer (FIG. 2) is provided with a user selectable sensitivity (160). Required operating parameters (172, 174, 176) are set in response to user selection of sensitivity (162) to permit measurement of an input light beam (12) at the selected sensitivity (160). Setting the required parameters (172, 174, 176) includes setting a required gain (172) of a video channel (48, 90, 92, 94, 96) to permit measurement of a specified maximum input signal and to provide the selected sensitivity (160), setting a required video bandwidth (174) of the video channel (48, 90, 92, 94, 96) to provide the selected sensitivity (160) at the required gain (172) of the video channel (48, 90, 92, 94, 96) and setting a sweep rate (176) to provide the selected sensitivity (160) at the required video bandwidth (174). When the normal bandwidth of the video channel (48, 90, 92, 94, 96) is not adequate to provide the selected sensitivity (160), the electrical signal is passed through a digital filter (100) having a filter coefficient (FILTER COEFFICIENT) set to provide the required video bandwidth (174). A peak detector (90) is incorporated in the video channel (48, 90, 92, 94, 96) to accurately measure signal amplitudes in a fast scanning condition.

Description: This invention relates to instruments for analyzing the spectrum of a light beam and, more particularly, to optical spectrum analyzers. Optical spectrum analyzers are used for analyzing the output light beams from lasers, light-emitting diodes and other light sources. Optical spectrum analyzers are particularly useful for analyzing light sources for optical telecommunication, where it is preferable to ensure that the optical carrier includes only a single, spectrally pure wavelength. In optical spectrum analyzers, the light intensity of a light beam is displayed as a function of wavelength over a predetermined wavelength range. Parameters of importance in optical spectrum analyzers include wavelength range, wavelength and amplitude accuracy, sensitivity, resolution, measurement speed, polarization insensitivity and dynamic range. Optical spectrum analyzers frequently use a diffraction grating for separating the light beam to be analyzed into its component wavelengths. The input light beam to be analyzed is collimated and is directed at the diffraction grating. The light beam is spatially dispersed by the grating, since different wavelengths are diffracted at different angles. The grating is rotated so that the dispersed light beam is scanned over a slit. The light that passes through the slit is detected to provide an output signal that represents amplitude as a function of wavelength. The width of the slit establishes the resolution of the monochromator. Prior art optical spectrum analyzers have used single stage monochromators as described above and two stage monochromators, wherein two monochromators operating in series provide improved performance. The output electrical signal is displayed as a function of diffraction grating position to provide an optical spectrum of the input light beam. Optical spectrum analyzers should operate over a wide range of parameter values for maximum versatility. For example, the selected wavelength span may range from a few nanometers to a few hundred nanometers. In some cases, the user may wish to accurately measure low amplitude signals, thereby requiring long sweep times. In other cases, accurate measurement of small amplitude signals may not be important to the user, and the sweep rate can be increased. Prior art optical spectrum analyzers have not adequately addressed the wide variety of user requirements. Optical spectrum analyzers with optical fiber inputs sometimes receive low power signal levels, which require long sweep times. Long sweep times are required because of the need for averaging to increase the signal-to-noise ratio. Frequently, the user selects a reference level to capture a peak of a signal and specifies a vertical amplitude scale on the optical spectrum analyzer display. However, without any other information, the instrument may be required to sweep very slowly in order to accurately display signals of the power level represented by the bottom of the display screen. The long sweep times are not acceptable to the user when the signal level represented by the bottom of the display is not of interest. Prior art optical spectrum analyzers usually require the user to select a detection or averaging method, or a mode where the instrument is either highly sensitive or less sensitive. Prior art systems have had several disadvantages. The user is not provided with information as to the minimum signal that is accurately displayed by the instrument. Furthermore, by providing only two or three instrument modes, the instrument is in most cases sweeping more slowly than necessary for the minimum detectable signal that is to be displayed. In addition, prior art systems have typically not provided information as to how the measurement is being taken. Without such information, erroneous measurements can occur. It is a general object of the present invention to provide improved methods and apparatus for optical spectrum analysis. Summary of the Invention According to an aspect of the invention, there is provided a method and apparatus for optical spectrum analysis using a peak detector to acccurately measure signal amplitudes during fast scanning. An optical spectrum analyzer for spectrum analysis of an input light beam comprises means for performing a wavelength scan of the input light beam to provide an electrical signal that represents the intensity of the input light beam as a function of wavelength, means for detecting a peak value of the electrical signal during each of a plurality of predetermined intervals, means for measuring the peak values during each of the predetermined intervals, and means for displaying the peak values as a function of wavelength to provide an optical spectrum of the input light beam. Brief Description of the Drawings For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the accompanying drawings which are incorporated herein by reference and in which: FIG. 1A,B is a pictorial block diagram of a double-pass scanning monochromator for use in an optical spectrum analyzer; FIG. 2 is a block diagram of an optical spectrum analyzer incorporating the present invention; and FIG. 3A-D shows the display screen and user interface of the optical spectrum analyzer. Detailed Description of the Invention An important component of an optical spectrum analyzer is a monochromator for separating an input light beam to be analyzed into its wavelength components. A double-pass scanning monochromator 8 is shown pictorially in FIG. 1. An input optical fiber 10 directs an input light beam 12 through a lens 14 to a diffraction grating 16. The input optical fiber 10 acts an input slit or aperture of the system. The input light beam 12 is collimated by the lens 14 prior to incidence on diffraction grating 16. The diffraction grating 16 diffracts the input light beam 12 to produce a spatially dispersed light beam 20. Since the diffraction grating 16 diffracts different wavelengths at different angles, as is well known, the input light beam 12 is spatially separated into its component wavelengths. The spatially dispersed light beam 20 passes through lens 14 and is reflected by a mirror 22 to an aperture plate 23 having a slit 24 that determines the resolution of the monochromator. The dispersed light beam 20 is focused by lens 14 at the plane of slit 24. The slit 24 passes a selected portion of the dispersed light beam 20. Slits 24 with different widths are used to provide different resolutions. The slit 24 acts as a spatial filter of the dispersed light beam 20 to produce a filtered light beam 30. The filtered light beam 30 is reflected by a mirror 26 through a half-wave plate 32 and lens 14 to the diffraction grating 16. The filtered light beam 30 is collimated by lens 14 prior to incidence on grating 16. Filtered light beam 30 is directed to diffraction grating 16 such that both time and wavelength dispersion are collapsed by diffraction grating 16 to form an output light beam 36. The output light beam 36 is reflected by a mirror 40 to an input end of an output optical fiber 42. The output light beam 36 is focused by lens 14 on the input end of optical fiber 42. The output light beam 36 is carried by optical fiber 42 to a photodetector 46 which converts the output light beam 36 to an output electrical signal. The output electrical signal is amplified by an amplifier 48. The diffraction grating 16 is rotated about an axis of rotation 50 by a diffraction grating motor 52 to provide scanning of the spatially dispersed light beam 20 with respect to slit 24. The axis of rotation 50 is selected such that the direction of scan is the same as the axis of dispersion. The light passing through slit 24 represents a scan across a selected wavelength range of the input light beam 12. The width of slit 24 determines the system resolution. The position of diffraction grating 16 is controlled in a servo loop to provide highly accurate scanning and highly accurate diffraction grating position information. A sweep signal is input through a summing unit 54 and a motor interface 56 to diffraction grating motor 52. The rotational position of diffraction grating 16 about axis 50 is sensed by an optical position encoder 58 having high resolution. The resolution of the position information is further increased by an interpolator 60 which outputs diffraction grating position information. The diffraction grating position information is input to summing unit 54. The difference between the sweep signal, which represents desired diffraction grating position, and the sensed grating position is an error signal used to drive diffraction grating motor 52. Further details regarding the monochromator 8 are disclosed in application (Assignee's Docket No. 191448, filed concurrently herewith and entitled "Optical Spectrum Analyzer Having Double-Pass Monochromator"). A block diagram of an optical spectrum analyzer is shown in FIG. 2. The photodetector 46 converts the output light beam on optical fiber 42 to an electrical signal. The photodetector 46 is typically a photodiode. The amplifier 48 includes a low noise transimpedance amplifier 70. Feedback resistors 72, 74, ---76 are selectably connected between the output and the inverting input of amplifier 70 by switches 82, 84 ---86, respectively, in response to gain control signals. The gain settings permit selection of different sensitivity levels as described below. The output of transimpedance amplifier 48 is input to a peak detector 90 and to a sample-and-hold unit 92. As described below, the sample-and-hold unit 92 is used for sampling the amplifier output signal when digital filtering is required, and the peak detector 90 is used when digital filtering is not required. An electronically controlled switch 94 connects the output of the peak detector 90 or the sample-and-hold unit 92 to the input of an analog-to-digital converter (ADC) 96. The signal samples from ADC 96 are input to a digital signal processor 106, which provides means for processing the ADC data by digital filtering means 100 or digital peak detection means 102. When digital filtering is required, the output of sample-and-hold unit 92 is connected by switch 94 to ADC 96. When digital filtering is not required, the output of peak detector 90 is connected to ADC 96, and digital peak detection is used. The elements between the photodetector 46 and the digital signal processor 106 constitute a video channel. A peak detector 90 is provided to allow accurate measurement of the signal amplitude during fast scanning of the monochromator, wherein the rate of change in amplitude of the signal exceeds the sample rate of the ADC 96. In this case, the output of the peak detector holds the maximum value of the signal during the time between ADC conversions. Each ADC value represents the maximum signal amplitude which occurred since the previous ADC value. The ADC sample interval is approximately 40 microseconds. In a preferred embodiment, an ADC 96 with two input channels is used. The second channel provides a separate peak detector 90, sample-and-hold 92, and switch 94 with gain preceding this circuitry. This additional channel is used to increase the measurement range of the ADC 96. The channel with the highest signal level within the range of the ADC is selected prior to each ADC conversion. The digital signal processor 106 provides appropriate signals to a display and user interface unit 108 for generating an optical spectrum display as described below. The digital signal processor 106 also provides the sweep signal to monochromator 8 and provides a filter coefficient to digital filter 100 to determine the filter bandwidth. The diffraction grating position information is connected from monochromator 8 to digital signal processor 106 to provide wavelength information for the optical spectrum display. A trace processor 107 combines the diffraction grating position information and the signal data to provide display data to display and user interface unit 108. The digital signal processor 106 can, for example, comprise a Motorola 56001 digital signal processor. The display screen and user interface of the optical spectrum analyzer are shown in FIG. 3. The amplitude of the input light beam is displayed as a function of wavelength on a display screen 118. A typical spectrum 120 is shown. The instrument provides a menu-based user input. Selector keys 122, 124, 126, etc. along the left edge of the screen permit selection of parameter categories such as wavelength, amplitude, marker, etc. When one of the parameter categories is selected, a menu appears along the right side of the screen permitting selecting of various subparameters using selector keys 130, 132, 134, 136, etc. Values of the subparameters can be entered with a numeric keypad 140. For example, when wavelength is selected with selector key 122, the menu on the right side of the screen permits selection of center wavelength, wavelet span, start wavelength, stop wavelength, etc. When amplitude is selected, the menu permits selection of reference level, sensitivity, dB per division (dB/Div), etc. A user interface of the type shown in FIG. 3 is implemented in Models HP71450A and HP71451A, manufactured and sold by Hewlett Packard Company.

Claims: An optical spectrum analyzer for spectrum analysis of an input light beam (12) comprising: means (8) for performing a wavelength scan (170) of said input light beam (12) to provide an electrical signal that represents the intensity of said input light beam (12) as a function of wavelength; characterized by: means (90) for detecting a peak value of said electrical signal during each of a plurality of predetermined intervals; means (96,106) for measuring said peak values of said electrical signal during each of said plurality of predetermined intervals; means (118) for displaying said peak values as a function of wavelength to provide an optical spectrum (120) of said input light beam (12). An optical spectrum analyzer as defined in claim 1 wherein said means (96, 106) for measuring comprises an analog-to-digital converter (96). A method for spectrum analysis of an input light beam (12) using an optical spectrum analyzer, said method comprising the step of: performing a wavelength scan (170) of said input light beam (12) to provide an electrical signal that represents the intensity of said input light beam (12) as a function of wavelength; characterized by the steps of: detecting a peak value of said electrical signal during each of a plurality of predetermined intervals; measuring said peak values during each of said predetermined intervals; and displaying said peak values as a function of wavelength to provide an optical spectrum (120) of said input light beam (12).

Inventors: AZARY ZOLTAN D, STIMPEL JAMES ROY, WARWICK STEVEN DAVID, WILDNAUER KENNETH RICHARDS, AZARY, ZOLTAN D., STIMPEL, JAMES ROY, WARWICK, STEVEN DAVID, WILDNAUER, KENNETH RICHARDS
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1992
Main classification: G01J3
IPC codes: G01J3