Migration from RF to Optical platforms

Girish Baliga, Marketing Program Manager, Digital, Wireless & Aerospace Defense test, Keysight Technologies

The new paradigm in Space

optical-fibreThe space industry is experiencing disruption not seen since the original space race of the 1960’s. Numerous new companies are gearing up to provide space-based capabilities with new risk profiles, increased volumes and decreased costs. Established companies are adapting to take advantage of the “NewSpace” opportunity. Lower development, production and launch costs are critical keys for success for all.

Advancement in space technology & development of more sophisticated space-based instrumentation has opened a new chapter for optical space-based communication. Due to increasing demands for high data rate and large communication capacity, researchers are actively working to build all optical communication architecture that includes ground-to-satellite optical communication links which are connected to satellite optical network and satellite-to-ground optical links.

Optical communication has gained significant importance owing to its unique features: large bandwidth, high data rate, easy & quick deployment, less power and low mass requirements. FSO communication uses optical carrier in the near infrared (IR) band to establish either terrestrial links within the Earth’s atmosphere or inter-satellite/deep space links or ground-to-satellite/satellite-to-ground links as shown in figure 1.

Figure 1
Figure 1
Advantages of Optical Communication over RF Communication

High bandwidth:

It is a well-known fact that an increase in carrier frequency increases the information carrying capacity of a communication system. In RF and microwave communication systems, the allowable bandwidth can be up to 20% of the carrier frequency. In optical communication, even if the bandwidth is taken to be 1% of carrier frequency (≈1016 Hz), the allowable bandwidth will be 100 THz. This makes the usable bandwidth at an optical frequency in the order of THz which is almost 105 times that of a typical RF carrier.

Unlicensed spectrum:

In the RF system, interference from adjacent carrier is the major problem due to spectrum congestion. This requires the need of spectrum licensing by regulatory authorities. But on the other hand, the optical system is free from spectrum licensing till now. This reduces the initial set up cost and development time.

High Security:

Optical communication cannot be detected by spectrum analyzers as optical laser beam is highly directional with very narrow beam divergence. Any kind of interception is therefore very challenging.

Immunity and reliability:

Fiber provides extremely reliable data transmission. It’s completely immune to many environmental factors that affect copper cable. It’s immune to electrometric interference and radio-frequency interference (EMI/RFI), crosstalk, impedance problems, and more. You can run fiber cable next to industrial solution without worry. Fiber is also less susceptible to temperature fluctuations than copper and can be submerged in water.

RF-over-Fiber (RoF) | The concept & typical use cases

Going back to basics, Lightwave transmission systems operate just below the frequency of visible light, in the part of the near infrared spectrum between 800 and 1600 nm.  Using the wavelength equation, you can calculate that the frequency of a 1550 nm wavelength signal is ~193 THz (~193,000,000,000,000 cycles/s).This is the wavelength used for long-haul telecom links. Note that even if you can use only 2 to 3% of the spectrum around that center frequency, there is more than 5,000 GHz of bandwidth available!

Wavelengths around 1310 nm are used for medium-haul, regional telecom links, and local area networks typically operate at 1310 or 850 nm.

In contrast to electromagnetic energy at lower frequencies, lightwave carriers are usually described by their wavelengths rather than frequencies, a holdover from physicists reporting emissions bands as spectral lines of a specific wavelength. Also, instrumentation like diffraction-grating spectrometers and interferometers resolve spectra using the physical basis of wavelength.

Lightwave digital communications systems transmit information by simply switching transmitters (lasers or LEDs) between two optical power levels, P1 and P0 , where P1 is the power generated when the light source is on, and P0 is the power generated when the source is off. Extinction ratio is the ratio of P1 to P0. Note that, especially in conjunction with lasers, P0 is not zero but rather slightly above the laser’s threshold. This is because lasers become slow below the threshold. Small variations in extinction ratio can have a large effect on the performance of digital communications systems.

Use cases | Modern Day Optical Communication System based on DWDM

Figure 2
Figure 2

Current DWDM systems as shown in figure 2 above typically combine 16 to 40 channels from client-layer network terminals (NTs) using a wavelength multiplexer (mux). At the receiving end, a de-multiplexer (demux) separates the channels and sends them to individual receivers in client-layer NTs.  Because they work only from about 1530 to 1565 nm (conventional C-band), and more recently also from about 1570 to 1620 nm (L-band), DWDM systems have been designed and optimized for one of these two wavelength ranges. This requires wavelength-converting transponders for each channel at both ends of the link.

On the transmission side, each transponder optically receives a traffic signal at any 1300 or 1550 nm wavelength, converts it to the electrical domain, and retransmits it using a 1550 nm band laser (for a C-band system). On the receiver side, a de-mux separates the various 1550 nm band signals, and the transponders regenerate the wavelengths and client traffic patterns (for example, SONET/SDH) that the various NTs are designed to receive. The wavelength-converting transponder allows DWDM operators to incorporate existing NTs or equipment from different vendors.

A typical use of Radio-over-Fiber(RoF) is data transmission between Control Station and Antenna via fiber link as shown in figure 3.

fig3Control Station transmits optical carriers (fo) modulated at RF (fC & data) over fiber links toward remote base stations. Photodiode PD converts the optical signal into an electrical RF signal (fC,2fC… nfC & data) RF signal is amplified and transmitted by an antenna (fC,2fC… nfC & data) .

Test scenarios for Optical Communication System leveraging Radio-over-Fiber(RoF)

When light travels through any component of a fiber optic network, it experiences some attenuation (loss of intensity). Insertion loss is the total optical power loss caused by the insertion of the component. The insertion losses of all optical components, including connectors and splices used in transmission links define the system’s power budget and margins. Therefore, it’s important to be able to measure insertion loss accurately.

Insertion loss measurement is always a two-step procedure: In the first step, calibration, the input power to the DUT, P1 (also called the reference power) is measured. In the second step, the DUT is inserted and the power is measured again. The insertion loss is the ratio of the power received after the device is inserted (P2) to the reference power (P1). This ratio is usually expressed as a positive number in dB, so an insertion loss as close to 0 dB as possible indicates optimal transmission. Depending on the type of component (for example, connectorised, pigtailed, flange mount or integrated optics), the measurement setup varies.

Spectral characteristic of the DUT can also affect the measurement setup. For broadband components such as connectors, measurement of the insertion losses at one wavelength is enough. However, for narrowband components such as filters, multiplexers and demultiplexers, it is usually necessary to measure the insertion loss at several different wavelengths as shown in figure 4 below.

Figure 4
Figure 4

Every optical component has at least one optical interface. When light hits this interface some portion of it is reflected back. The ratio of the incident optical power to the reflected power is the return loss, which is usually expressed in dB.

A common technique for measuring return loss is called optical continuous wave reflectometry (OCWR). It is performed by sending a continuous wave (CW) optical signal to the DUT and measuring the total reflected CW power. A patch cord with a “termination coil” ensures that all reflected energy is from the DUT. A directional coupler is used to separate the reflected power from the incident power and send it to a detector for measurement as shown in figure 5 below.

Calibration of the measurement system is achieved by replacing the DUT with a device that has a known reflection. A commonly used reflection reference is Fresnel reflection from the fiber-to-air interface found on a standard polished connector.

Figure 5
Figure 5

Optical spectrum analyzer measurement shown in figure 6 below is a typical 16-channel DWDM spectrum after it passes a few optical amplifiers. Each channel can carry separate data streams.

Note that the absolute power varies between different channels. However, the signal-to-noise ratio (or more specifically, the signal-to-ASE-power-density ratio) is almost constant, which is desired by system designers.

Figure 6
Figure 6

When the waveform passes from left to right, it can get degraded due to the combined effects of the laser, cable/connectors, and receiver as shown in figure 7 below. We can determine the overall effect on performance by convolving the responses of each of the constituent parts. The same information is available if we examine the frequency response of our network parts and then combine the result by multiplying their individual responses which is of course equivalent to the convolution of the time responses. Using a Lightwave Component Analyzer as shown in figure 7 below allows us to measure the frequency responses of our network components separately and then combined them into complete response by multiplying their individual responses.

Figure 7
Figure 7
Summary

By fully exploiting the terabit capacity of optical systems, courtesy, the very nature of optical physics, it can provide an interesting proposition for Space designers to switch from RF domain to optical domain.

For more on this topic & exciting application notes and information related to Space test technology & others, contact your nearest Keysight Technologies representative at *Toll free no.* or tm_india@keysight.com

References:

http://literature.cdn.keysight.com/litweb/pdf/5991-1802EN.pdf

http://literature.cdn.keysight.com/litweb/pdf/5968-5328E.pdf

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