How Tunable Lasers Are Enabling High-Speed Communications

PublishTime： 2024-04-01 Article Source：Keysight Blogs

Do you know that all the text and images you are seeing right now were, at some point, transmitted as light signals over optic fibers? Fiber optic communication networks are the heart of all the communication networks in data centers and inter-continental undersea cables that enable you to read this article, receive calls and emails on your smartphone, and watch your favorite videos.

Lasers are essential to the operations and testing of such networks. As the world's data volume grows, pushing more data over existing fibers becomes imperative. This is where tunable lasers can help improve network capacity and performance.

In this article, find out what are tunable lasers, where they're used, and how they operate.

What are tunable lasers?

Lasers are monochromatic, meaning that they emit light at a particular wavelength. For regular lasers, this wavelength is fixed.

In contrast, a tunable laser is an optical source whose emission wavelength can be selected from a wide range and dynamically changed to meet the needs of different applications.

For example, the Keysight N7776C tunable laser source can emit at any wavelength in its tuning range. That is, for example, the 1,240-1,380 nanometer (nm) O-band, the 1,340-1,495 nm E-band, or the 1,450-1,650 nm bands covering the C-, L- and U-bands.

At each wavelength in the tuning range, a tunable laser achieves coherence and narrow spectral linewidths of less than 500 MHz to as low as a few thousand Hertz (kHz). This is remarkable because regular lasing itself is a complex phenomenon requiring precise engineering, and tunable lasers go a step further by lasing coherently at any wavelength within their tuning range.

What are the key applications of tunable lasers?

Some common applications of tunable lasers are explained below.

High-speed communication networks

Modern data centers and 5G/6G mobile networks require high-speed fiber optic networks that can reach speeds of 400 gigabits to tens of terabits per second.

To accommodate thousands of concurrent network users on a single fiber, they use techniques like dense wavelength division multiplexing. Each data channel, itself aggregating the data of hundreds of users, is transmitted at a different wavelength on the same fiber.

Tunable lasers are essential for such high-speed, high-throughput multiplexing at low costs. Laser sources operating within the C-band and L-band achieve optimal transmission properties like low loss and good signal-to-noise ratios. Combined with arrayed waveguide grating techniques, they help modulate the bandwidth demand, dynamically change wavelengths, and reconfigure optical channels on demand.

Optical equipment testing

Tunable lasers are used with optical power meters and light polarization controllers to test a variety of photonics components and phenomena, like:

· measuring the wavelength filtering slopes of optical filters for multiplexing and demultiplexing in fiber optic networks

· testing the isolation of optical channels

· characterizing polarization dependence

· measuring insertion loss

· measuring the reflectivity of multiplexers, demultiplexers, channel interleaves, and wavelength-selective switches

All these are used in fast fiber optic networks that take multiple signals and merge or split them. These high-speed signals in modern networks change very quickly and can be easily thrown off by even minor imperfections in components.

Therefore, manufacturers must comply with very precise specifications. Tunable laser sources are indispensable for such precise quality control.

Tunable lasers also play a pivotal role in optical-sensing applications, where devices like an interferometer rely on their precise wavelength control.

Safety inspections

Tunable laser spectrometers are being used in safety equipment to check for leaks in pipelines carrying natural gas.

Medical imaging

Optical coherence tomography is a non-invasive imaging technique that provides high-resolution pictures of the internal microstructure of materials and biological tissues.

Material characterization using spectroscopy

Tunable layers, with their high resolutions and precise wavelength tuning, enable accurate substance identification and material analysis using Raman spectroscopy.

When light interacts with a material, it can be absorbed, emitted, or scattered. Each substance has a unique spectral fingerprint of such interactions.

A tunable laser source is configured to emit at specific wavelengths to selectively probe different aspects of a sample, such as identifying chemical bonds or concentrations of certain elements.

For such applications, continuous-wave tunable lasers that emit constant light without interruption are preferred over pulsed lasers because a steady uniform signal is essential for high-precision measurements.

Nanosecond-pulse tunable lasers are especially valuable in time-resolved spectroscopy and laser-induced breakdown spectroscopy, where brief, intense pulses allow for the study of rapid physical, chemical, or biological processes and materials analysis with minimal sample damage.

What are the types of tunable lasers?

There are multiple axes of categorization for tunable lasers.

Based on their construction and operating principles, tunable lasers can be:

· Semiconductor lasers: They are widely used for their compact size and efficiency in electronic devices and communication systems. Tunable diode lasers are a type of semiconductor lasers with compact form factors and efficient electro-optical conversion. If built based on an external cavity, continuous tuning ranges beyond 10% of the center wavelength can be achieved.

· Solid-state lasers: Solid-state ones like sapphire lasers use sapphire as the gain medium with dopants such as titanium to achieve wide tuning ranges and high beam quality, making them useful for high-resolution microscopy and spectroscopy.

· Distributed feedback (DFB) lasers: The lasing medium is integrated with a periodic structure that serves as a diffraction grating. The grating's pitch or temperature can be adjusted to change the emission wavelength. DFB lasers provide stable output with a narrow linewidth suitable for fiber optic communications. They offer a narrow tuning range.

· Vertical cavity surface emitting lasers (VCSEL): They are semiconductor lasers characterized by vertical emission of the laser beam instead of the more typical edge emission of conventional laser diodes.

· Dye lasers: They can operate over a broadband range of wavelengths, making them exceptionally versatile tunable light sources for spectroscopy and chemical analysis.

· Fiber lasers: They use optical fibers to guide the light and to form a resonator.

Alternatively, based on the resolution of their emission wavelengths, we can categorize them as:

· Single-mode lasers: These are the most precise type with exceptionally narrow linewidths (less than a GigaHertz to as low as a few Hertz).

· Broad-linewidth lasers: Their wide linewidths enable them to be constructed at lower costs for a wide range of applications, like detecting chemicals and pollutants.

How do tunable lasers work, and what are the mechanisms for changing wavelengths?

· A tunable laser source contains the following elements essential for lasing:

· A gain medium contains the electrons that emit photons when they change energy states.

An optical cavity resonator mechanism reflects the photons back and forth to achieve stimulated emission, acting like an amplifier for the optical signal.

As hinted in the previous section, there are many mechanisms to achieve tunable wavelengths.

· Current injection tuning: For semiconductor lasers like VCSELs and DFBs, varying the injection current changes the gain profile and refractive index, tuning the emitted wavelength.

· Quantum well intermixing: For semiconductor lasers, this involves modifying the bandgap energy of the quantum wells to change the emission wavelength. This can be done during fabrication or post-fabrication through techniques such as impurity diffusion or ion implantation.

· Mechanical tuning: Diffraction gratings and prisms are rotated or translated to change the feedback of specific wavelengths into the laser cavity. They often use micro-electro-mechanical systems (MEMS) for microscopic tuning of components.

· Cavity length adjustment: The physical length of the laser resonator can be altered, which changes the resonant modes and, consequently, the laser wavelength.

· Liquid crystal tuning: Liquid crystal elements can be used to alter the polarization of the beam, allowing certain wavelengths to be selectively amplified or suppressed.

· Thermal tuning: By adjusting the temperatures of the gain medium or the optical components in the laser cavity, the refractive index changes and influences the wavelength.

· Nonlinear optical techniques: Nonlinear optical crystals can generate light at different frequencies by mixing pump photons and signal photons of lower frequencies.

· Raman shifting: Using the Raman effect in gasses, liquids, or solid mediums, the wavelength can be tuned to longer wavelengths (Stokes shift) or shorter wavelengths (anti-Stokes shift).

· Mode-hop-free tuning: Some of the above tuning mechanisms exhibit mode hopping, where the wavelength jumps between discrete values in the laser's gain profile. Mode-hop-free tuning ensures seamless wavelength transitions and avoids sudden jumps that can disrupt sensitive measurements or communications.

In addition to the primary tuning mechanism, a tunable laser source contains the following elements essential for lasing:

· Optical amplifiers: They boost the emitted signal power without altering their properties.

· Optical oscillators: They generate coherent light through optical feedback and amplification.

What are the key specifications for tunable lasers?

Regardless of the tuning mechanism, some key characteristics of a tunable laser module or laser source are:

· wavelength tuning range, which is the span of wavelength range over which it can operate

· wavelength resolution, which is a measure of how precisely their frequencies can be adjusted specified in nanometers (nm) or picometers (pm), alternatively in gigahertz (GHz) or megahertz (MHz)

· optical output power, typically in decibel-milliwatts (dBm)

· sweep speed, which is the rate at which the laser can change its wavelength in nm per second

What are some disadvantages and challenges of tunable lasers?

Tunable lasers have their pros but also exhibit some common challenges that may turn into disadvantages compared to fixed-wavelength lasers:

· Achieving narrow effective linewidths and low noise requires sophisticated optical, electronic, and mechanical designs.

· Maintaining stable sufficient power output over the entire tuning range often requires innovative engineering.

· The reliability and consistency of tunable lasers are highly dependent on the quality of their driver circuit, which must deliver stable and precise electrical currents, and on how well the laser cavity is controlled mechanically and electronically, to maintain optimal laser performance.

What are some emerging trends and future innovations in tunable lasers?

Emerging trends in tunable laser technology include the following:

· In addition to III-V semiconductors like gallium arsenide and indium phosphide for their direct bandgap properties, tunable laser sources are using new structures and novel alloys for enhanced performance, new wavelength ranges, and improved tunability.

· The development of new configurations—such as quantum-confined systems like quantum dots and quantum wells—offer higher output power, better wavelength resolutions, greater flexibility, and more integration possibilities for various optical systems.

· The miniaturization of tunable cavities offers faster thermal tuning and higher-density packaging in communication networks.

Tunable lasers may be the future of all laser technology

In this article, we learned about tunable lasers and how they work. The cost savings and versatility of tunable lasers make them no-brainers for any application that requires lasers.

Currently, tunable laser technology still has some disadvantages and challenges. However, with rapid advances in semiconductor research, quantum engineering, and MEMS, it appears that its current problems will be successfully solved over the next decade, making it a ubiquitous component of communication networks of the future.

Keysight's state-of-the-art high-precision benchtop tunable laser measurement products and tunable laser sources enable the testing and measurement of high-speed optical communication networks, scientific instruments, and fiber optic systems.

Keysight tunable lasers — like the top line N7776C, the value line N7778C, or the basic line N7779C — for testing and qualifying your optical systems and communication networks.

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