Optical Spectrum

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

The optical spectrum (or emission spectrum) of a light source or some beam contains information on how the optical energy or power is distributed over different wavelengths. Usually, it is presented in the form of a diagram where some spectral quantity is plotted as a function of the wavelength or optical frequency. The plotted quantity may e.g. be a spectral flux (in units of W/Hz or W/nm), a spectral intensity (in W sr⁻¹ Hz⁻¹ or W sr⁻¹ nm⁻¹) or spectral radiance (in W sr⁻¹ m⁻² Hz⁻¹ or W sr⁻¹ m⁻² nm⁻¹), but in many cases optical spectra are presented without an absolute scale. In some cases, one uses a (calibrated or uncalibrated) logarithmic scale, e.g. with units of dBm/nm.

As an example, Figure 1 shows the numerically simulated optical spectrum of a supercontinuum source, which is a highly polychromatic light source. In contrast to that very broad spectrum, the optical spectrum of a single-frequency laser source is often characterized by a very narrow spectral line – in extreme cases, with a linewidth of the order of 1 Hz, corresponding to only ≈ 3 ·10⁻¹² nm (for 1 μm center wavelength). Other lasers have a spectrum consisting of multiple lines, and some (particularly mode-locked lasers for ultrashort pulses) can have a large spectral width of 100 nm or more with a frequency comb structure.

Optical spectra can be recorded with different types of spectrometers (e.g. spectrographs), which greatly differ in terms of the covered spectral range and the spectral resolution.

The optical spectrum is intimately related to the temporal coherence properties of the light. For example, the temporal coherence function fully determines the spectrum. The optical spectrum is also related to the Fourier transform of the electric field (although the latter is in most cases not directly accessible). Therefore, it is also called the Fourier spectrum of the optical field.

Optical Bandwidth

The optical bandwidth is essentially the width of the optical spectrum. There are different definitions, but the full width at half maximum (FWHM) is often used.

Optical Spectra with Line Structures

Some optical sources such as incandescent lamps, light-emitting diodes or superluminescent sources have a very smooth optical spectrum. However, other sources can have spectra consisting of closely spaced narrow lines, which can be resolved only with a spectrometer having a sufficiently high spectral resolution (small resolution bandwidth). In the case of a continuous-wave laser with multimode emission, but only on fundamental resonator modes, the lines in the spectrum are approximately (but not precisely) equidistant, with a spacing which typically corresponds to the inverse resonator round-trip time and is in the megahertz or gigahertz region. (The round-trip time can be frequency-dependent due to chromatic dispersion, and nonlinearities may also affect the mode frequencies.) If the laser also emits on higher-order transverse modes, there are additional intermediate lines (related to the Gouy phase shift), so that the line spectrum is denser and not equidistant. Any mode-locked laser, however, produces a frequency comb spectrum with exactly equidistant lines, apart from some laser noise, which is often very weak.

Measuring Optical Spectra

Optical spectra can be recorded with instruments which are called optical spectrum analyzers. They are often a kind of spectrographs, but there are other kinds of spectrum analyzers based on completely different operation principles. For example, there are interferometric devices, using either Michelson interferometers (→ Fourier transform spectroscopy) or on Fabry–Pérot interferometers.

It is also possible to combine spectral analysis with imaging – see the articles on multispectral imaging and hyperspectral imaging.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 14 suppliers for frequency comb sources. Among them:

Menlo Systems

As the pioneer in the optical frequency comb technology, Menlo Systems offers a full product line from the compact and fully automated SmartComb to the ultra-low noise optical frequency comb FC1500-ULN^plus. Our patented figure 9^® mode locking technology ensures lowest phase noise and long-term reliable operation.

Octave Photonics

Octave Photonics offers electro-optic frequency combs at 5 to 30 GHz repetition rates. These combs can be fully stabilized using Octave's nanophotonic chip technology.

Menhir Photonics

The MENHIR-1550 SERIES is the first 1-GHz turn-key femtosecond laser at 1550 nm allowing for ultra-low noise optical frequency comb with wide comb-spacing. Hermetically sealed and all-in-one (laser and electronic is one box), Menhir Photonics’ products have been designed to achieve lowest phase noise combined with high-reliability and robustness.

Alpes Lasers

Alpes Lasers offers mid-IR frequency combs centred around 6 μm or 8 μm. The QCL comb is a stand alone device as it integrates the pump laser and the microcavity in its waveguide contrarily to other comb technologies. This makes it a very compact comb source. Being based on QCL technology, comb devices can be manufactured over all the MWIR and LWIR.

TOPTICA Photonics

TOPTICA’s Difference Frequency Comb (DFC) is a compact, robust and high-end solution featuring turn-key operation in a 19 inch format. The patented CERO technology uses difference frequency generation to intrinsically fix the ν_CEO at 0 Hz. This allows for one control loop less compared to standard f–2f approaches, resulting in lowest CEP noise and a decoupling of ν_CEO and f_rep. Thus, the DFC is the number one choice for anyone looking for high-end performance combined with a high level of robustness.

K2 Photonics

The K2-1000-mini is a next-generation 1 GHz repetition rate femtosecond laser, engineered for high-power, ultra-low noise frequency comb generation. This delivers massive increase in power per comb line, significantly boosting signal-to-noise ratio for precision metrology applications. Designed for turnkey integration, K2-1000-mini features:

• High peak power for efficient supercontinuum generation across a broad spectral range.
• Compact, quasi-monolithic design, ensuring robustness and high long-term stability.
• Dual-comb modelocking option from a single cavity.

Proprietary dual-comb modelockng technique eliminates active stabilization, simplifying system integration and enables direct dual-comb spectroscopy, reducing complexity.

With its GHz-class performance, compact form factor, and unmatched dual-comb capability, the K2-1000-mini sets a new standard for frequency comb sources in scientific and industrial applications.

Bibliography

(Suggest additional literature!)

Questions and Comments from Users