technology photodiodes
requirements for next-generation Gbit/s wireless networks.
The simplest way to rescue us from this famine is to boost up wireless carrier frequencies. Several currently ‘unlicensed’ (sub-) millimetre wave (MMW) bands are attracting attention for this purpose, including the V- band (60 GHz), D-band (120 GHz), and bands beyond 300 GHz. However, (sub-) MMW signals suffer substantial propagation loss, both in free-space and in transmission lines. This weakness, and their inherent straight-line path of propagation, hamper connections and synchronization between different parts of the communication system.
A promising way to overcome this problem is the MMW- over-fibre (MOF) technique (see Figure 1 for two examples of MMW over-fibre communication systems). This approach has three key strengths: It employs flexible optical fibre, rather than rigid MMW waveguides; propagation losses fall by several orders of magnitude (in MMW waveguides the loss is typically 0.05 dB/cm at 100 GHz, compared with typically 0.1 dB/km for MMW carrier-wave signal distribution); and network coverage is increased significantly. However, there is a tremendous difference in frequency between the 1.55 µm signals used in optical networks, which have a frequency of 190 THz, and MMW signals that have wavelengths of a few millimetres and frequencies of tens or hundreds of gigahertz.
MMW-over-fibre solutions
The first example provided in Figure 1 shows setups for additional electrical-to-optical and optical-to-electrical conversion in the central office and base station of an MOF system. The electrical-to-optical conversion in the central office usually employs 1.55 µm lasers and optical modulators. To let the optical wave have the desired MMW envelope for remote distribution through a low-loss fibre to several base stations, the electrical MMW is used for the input signal. This approach eliminates the huge propagation loss of the MMW signal that occurs along an electrical transmission line or in free-space. This optical signal is returned to the electrical domain at the base station. Down-conversion of the incoming optical signal extracts the MMW envelope, which is radiated over the last-mile to the end-user through an antenna.
A high-speed photodiode that can operate at the (sub-) MMW regime is the key component for converting the optical MMW envelope into an electrical MMW signal. Ideally this device also has a high output saturation power, so that it can generate high MMW power under intense optical power injection.
When these high-power MMW PDs are used in partnership with a high-power, erbium-doped fibre amplifier (EDFA), it is possible to minimize the burden imposed on the limited gain, noise, and saturation power performance of the next-stage MMW amplifier for wireless data
Figure 1.A MMW-over-fibre communication system promises to improve wireless data transfer
rates.Such a system can feature a common optical local oscillator MMW source shared by: (a) different base stations and (b) different electrical local oscillator MMW sources installed in each base station
March 2012
www.compoundsemiconductor.net 27
transmission. What’s more, the optical signal processing technique can deliver a photo-generated MMW signal of superior quality to that generated from a chain of MMW mixers and amplifiers.
It is also possible to build an MOF system that does not require a MMW photodiode (see Figure 1(b)). In this case, the optical fibre links are used to distribute the optical data signals from the central office to each base station. In the base station, incoming optical data is transformed into electrical MMW signals for radiation to the end-user with an oscillator, mixer, amplifier, and antenna operating in the MMW bands.
With this approach, the photodiode only converts low- frequency optical data; there is no optical MMW signal in this system. However, this benefit has to be weighed against more expensive and complex base stations, which must be synchronized and share the same optical MMW signal. Greater expense stems from the high-cost MMW ICs, which include the elements mentioned above, such as a mixer, oscillator, and amplifier for the MMW bands.
In addition, it is challenging to synchronize the different MMW oscillators at different base stations. This may be an issue for mobile users roaming among different base stations. Given this complexity, it is clearly better to build an MOF system that incorporates a MMW photodiode.
This device can be integrated with a MMW antenna to form a photonic transmitter (PT) at the base station of a high-performance MMW photonic-wireless link that also features a high-quality optical MMW source in the central office.
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