High-speed Measurement Technologies of AQ6370C Optical Spectrum Analyzer

Manabu Kojima¹ Tohru Mori¹ Tsutomu Kaneko¹ Toshikazu Yamamoto¹ Atsushi Horiguchi¹ Gentarou Ishihara¹

The optical spectrum measurement is indispensable to the wavelength division multiplexing (WDM) technology that is increasingly being used in optical communications. Optical spectrum analyzers, which measure and analyze the optical spectrum of light that travels through optical fibers, are required to have high performance, functionality, and throughput as sophisticated optical communication devices become available at low prices. To respond to these demands, Yokogawa has developed the AQ6370C optical spectrum analyzer with high accuracy, resolution, dynamic range, and fast measurement of 0.2 sec for a 100-nm span. This paper describes the optical spectrum measurement technologies in the AQ6370C.

1. Optical measuring instruments Division, Yokogawa Meters & Instruments Corporation

INTRODUCTION

Figure 1 External view of AQ6370C

The amount of information transmitted over the Internet has been increasing along with the spread of smart phones and the development of cloud computing services, and telecommunications carriers are reinforcing their network infrastructure accordingly. Optical spectrum analyzers are widely used for evaluating fiber-optic communication systems and optical communication devices, and serve as important measuring instruments for high-speed, large-capacity network infrastructure. To ensure transmission quality of networks, optical components used in the system must have higher quality, inevitably requiring optical spectrum measurement.

Meanwhile, shorter measuring time is a significant requirement because the man-hours required for inspection directly affect the cost of products. The AQ6370C shown in Figure 1 features the highest optical performance and measurement speed in its class. This paper introduces the development, performance, and functions of the AQ6370C.

OPTICAL SPECTRUM ANALYZER

Measurement Principle

Figure 2 Principle of optical spectrum analyzer

Optical spectrum analyzers spectroscopically measure and analyze optical power at each wavelength. Figure 2 shows the principle of optical spectrum measurement with an optical spectrum analyzer.

The wavelength of the light input to the optical bandpass filter is restrained to be within a narrow wavelength slot, and then the passed light is converted into electrical signals at the photodiode (O/E converter). An optical spectrum is obtained by plotting the electrical signals while sweeping the center wavelength of the optical bandpass filter. The optical bandpass filter is a mechanical device, equipped with an optical prism called a monochromator¹. Higher-performance optical spectrum analyzers are usually equipped with an optical bandpass filter with narrower bandwidth and steeper roll-off. In addition, the wavelength accuracy can be higher when the accuracy of controlling the center wavelength of the optical bandpass filer is higher.

Performance indexes

Key performance indexes for optical spectrum analyzers are listed below:

• Wavelength accuracy
  Capability to stably measure a wavelength with absolute accuracy
• Resolution
  Capability to distinguish two close line spectrums with wavelengths of λ and λ + Δλ
• Optical dynamic range (steepness characteristic of a filter)
  Capability to suppress the optical power of light with wavelengths close to a targeted wavelength when filtering measured light
• Speed
  Wavelength sweeping speed

DEVELOPMENT OF AQ6370C

Figure 3 Evaluation example of AQ6370C's wavelength certainty

Technologies for High Wavelength Certainty

As described in the Measurement Principle section, the more precisely the center wavelength of the optical bandpass filer is controlled, the more certain a wavelength can be achieved. A monochromator rotates a chromatic dispersion element, or diffraction grating, to sweep wavelengths. Thus, the rotating angle of the diffraction grating corresponds to the wavelength to be measured.

The AQ6370C uses a servo system, where a DC motor drives the diffraction grating and an encoder detects the rotating angle, to perform feedback control. An optical encoder is used, and detected electrical pulses are multiplied by several thousands to precisely control the rotating angle of the diffraction grating with a resolution of 0.2 or smaller arc seconds. The AQ6370C corrects any slight linearity errors caused by electrical multiplication to ensure a wavelength certainty within ± 0.01 nm. Figure 3 shows an evaluation example of the wavelength certainty of the AQ6370C. The vertical axis is the measurement error of wavelength and the horizontal axis is the wavelength.

Technologies for High Resolution

Figure 4 shows a basic configuration of a monochromator. The monochromator has a planar diffraction grating and two concave mirrors: a collimating mirror and a focusing mirror.

The incident light from the optical fiber is made parallel by the collimating mirror, and diffracted at the diffraction grating. Then this light reflects on the focusing mirror and dispersively forms a spectrum on the slit plane (in the horizontal direction in Figure 4 ). Only the light with the wavelength that focuses at the slit can pass it through. The resolution bandwidth (RBW) of the monochromator is given by the following equation (1).

Where, d is the grating pitch of the diffraction grating, m is the diffraction order, and β is the angle between the outgoing beam and the normal line to the reflection plane. It is clear from the equation that a longer focal length f of the focusing mirror or narrower width ε of the slit can obtain a better RBW². However, the focusing mirror with a longer focal length makes the monochromator larger. Thus, the additive-dispersion double path method is generally used to reciprocate light in the monochromator. Yet, high resolution cannot be obtained with a beam of large diameter focused on the slit. Yokogawa's original design of the collimating mirror and focusing mirror minimizes the aberration of the optical components to obtain a focused beam with a small diameter. As a result, the beam can focus on the slit with a width of 20 μm or narrower, leading to a resolution of up to 0.02 nm.

Figure 4 Basic configuration of monochromator

Technologies for Wide Dynamic Range

As described earlier, a monochromator works as an optical bandpass filter. The bottom-left illustration in Figure 5 shows the filtering characteristics with a single bandpass filter. The difference between the pass band and the stop band (optical dynamic range¹) is about 40 dB. As shown in the bottom-right in Figure 5, adding another filter will dramatically improve the dynamic range.

Connecting filters in cascade as described earlier is the basic design concept of the monochromator in the AQ6370C. Conventional double-pass (or multi-pass) monochromators have improved space efficiency. However, because of overlapping of the optical paths of the first and second filters in this design, undesirable optical elements such as scattered light generated at the first path enter the second one. This affects the dynamic range, and ideal performance cannot be obtained in a cascaded connection.

On the other hand, the new monochromator completely separates the first optical path from the second one by skillfully configuring optical parts, achieving performance close to the theoretical value in the cascade connection shown in Figure 6. In this system, wide dynamic range measurement, which is conventionally performed in a special measurement mode requiring a significant long time, can be achieved in a normal measurement mode, thus shortening the measuring time.

 

Figure 5 Improvement of dynamic range

Technologies for High-speed Measurement

The measuring time of optical spectrum analyzers depends on the rotational speed of the diffraction grating. The diffraction grating of the AQ6370C is driven by a servo system. By optimizing the motor design and using acceleration/ deceleration control, the AQ6370C achieves a sweep time of 0.2 seconds or less for a 10-nm wavelength range.

Figure 6 Dynamic range in the normal measurement mode of AQ6370C

The AQ6370C also achieves high-speed measurement of tunable lasers. Tunable lasers have been widely used as an optical source for large-capacity, long-distance communication in recent years. During quality inspection, it is crucial to measure how much the side mode of the main signal of the laser is suppressed. In this measurement, the spectrum including the main peak and side mode of the main signal of the laser needs to be measured at the same time. Therefore, it is important to measure with wide dynamic range and at high speed. Optical spectrum analyzers have an auto-range measuring function, which enables the measurement of optical power while automatically changing the gain of an internal amplifier depending on the input light power level. The AQ6370C achieves high-speed measurement even when in auto-range mode by optimizing the internal time constant when switching the gain and the rotational speed of the diffraction grating.

When used with the smoothing function, the AQ6370C can improve the S/N ratio especially when measuring side modes without requiring extra time. The smoothing function of the AQ6370C can automatically identify the location in the measured spectrum where the S/N ratio is insufficient and perform optimal smoothing. Thus, this function is effective when measuring the side modes of tunable lasers or distributed feedback (DFB) lasers.

Figure 7 shows an example of high-speed measurement of the spectrum of a DFB laser. The side mode of the laser is precisely measured in about 0.3 seconds.

Optical Input Section with Free-space Structure

When the light to be measured is input to the optical spectrum analyzer via the optical connector, the optical power level fluctuation when attaching and detaching the optical connector must be suppressed. When connecting an optical fiber, lower insertion loss is desirable for various applications even when connecting a multimode fiber such as GI62.5. Thus, Yokogawa uses a free-space structure for the optical input section, which has been highly evaluated by customers. This free-space structure is also implemented in the AQ6370C by optimizing the layout of optical components. This suppresses the optical power level fluctuation when attaching or detaching an optical connector, and multimode fibers can be connected with little insertion loss.

APPLICATIONS

Figure 7 Example of high-speed measurement of DFB laser

Typical applications of the AQ6370C are described below.

Analysis of DWDM/WDM Signals

Thanks to the wide optical dynamic range, the optical signal-to-noise ratio (OSNR) of the 50 GHz-spacing dense WDM (DWDM) transmission system can be precisely measured.

As the number of multiplexed signals increases, the OSNR cannot be accurately measured due to the limitation of the optical dynamic range of the monochromator. This requires choosing the special measurement mode (wide dynamic range mode) which makes the measuring time substantially longer. As the AQ6370C has a wide optical dynamic range, it can accurately measure OSNR at high speed in the normal mode even if the number of multiplexed signals increases. An example of analysis with the wavelength division multiplexing (WDM) analysis function of the AQ6370C is shown in Figure 8. The wavelengths, levels, spacing, and OSNRs of the WDM signal are all measured, and the results are displayed in the table.

Evaluating Optical Fiber Amplifier

With the noise figure (NF) analysis function, the AQ6370C can measure gain and NF, which are indexes for evaluating an optical fiber amplifier, for each channel at the same time. The output level of the amplified spontaneous emission (ASE) during the NF measurement contains the source spontaneous emission (SSE) of the WDM signal. The AQ6370C can remove this SSE component with the SSE suppression function to accurately measure NF values. In addition, the AQ6370C can graphically display the values of the gain and NF of each channel, thus the differences among gains of each channel (gain tilt) can be visually verified.

LINEUP FOR NON-COMMUNICATION BAND

Figure 8 Example of results of WDM analysis

Yokogawa has been accumulating technology for optical spectrum measurement in the optical fiber communication market, where high quality and reliability are demanded, and offers a lineup of products that employ these technologies for non-communication bands.

The AQ6373 with the wavelength range of 350 to 1200 nm is best suited for measuring laser spectra and transmittance of optical filters in the medical and biotechnology fields. With a high wavelength resolution of 0.02 nm, it also serves well in the industrial field for laser processing machines, etc., and in the consumer electronic field for laser projectors, etc.

The AQ6375 with the wavelength range of 1200 to 2400 nm is used to measure optical absorption for obtaining the distribution and concentration of greenhouse gases such as CO₂, SO₂, NO_x and methane, which are attracting attention as global environmental issues. The AQ6375 is also used in developing and manufacturing semiconductor lasers and fiber lasers which are used for sensing those gases.

 

CONCLUSION

This paper explained the technologies of the AQ6370C optical spectrum analyzer. The AQ6370C features high wavelength certainty, high wavelength resolution, wide dynamic range, and high measurement throughput. We hope that Yokogawa's technology for optical spectrum measurement will contribute to the expansion of optical fiber communications.

REFERENCES

1. Manabu Kojima, "Basic Fiber-optic Communication and Spectral Measurement," Transistor Gijutsu, Vol. 42, No. 7(490), 2005, pp. 215- 223 in Japanese
2. Tohru Mori, Tsutomu Kaneko, et al., "Development of Optical Spectrum Analyzers," Ando Technical Bulletin: Special Optical Measurement Edition, November, 2001, pp. 95-107